US stocks swung from losses to gains as investors struggled to evaluate the possibility of tougher regulation, higher taxes and oversight of areas such as AI and cryptocurrencies.
Stock futures in the US had initially declined after the results, with the tech-heavy Nasdaq 100 falling as much as 2.4% and S&P 500 futures down 1.4% as traders reacted to the likelihood of a "blue wave" sweeping Washington.
However, they clawed back those losses, with futures up 0.16% and 0.35% respectively, after Donald Trump accepted the election results and acknowledged that the war in Iran played a role in Republicans' losses.
A decisive win for the Democrats could exacerbate volatility in asset prices, especially in the artificial intelligence trade.
The party made gains in key data-centre states, where public opposition to the industry has been growing, including in Georgia and Texas, which could have implications for the AI buildout there.
"Those two states effectively make up about a third of all AI data centre spend and projects," said Tapas Strickland, chief market strategist at Moomoo ANZ in Sydney.
"The potential for it to slow is very much there, and that would obviously cascade through to the rest of the AI sector," he added. "I don't think it necessarily stops the whole AI train, but it does slow it down."
Shares in tech giants such as Microsoft, Amazon and Nvidia were all trading lower in extended trading.
The outcome may also have broad implications across the economy - affecting everything from defence spending to the regulation of financial assets.
US midterms - live updates as results come in
A very different Congress
Donald Trump's Republican Party has controlled both chambers of Congress during the first two years of his second term.
The Democrats have now taken both the House of Representatives and the Senate.
A Democratic-led Congress could exert greater scrutiny over artificial intelligence companies as public opposition to data centres grows.
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The US elections have weighed heavily on the markets for weeks - as well as concerns that heavy spending on artificial intelligence is yet to provide sufficient returns.
Rising interest rates and sticky inflation, which has been above the Federal Reserve's 2% target for several years, are also playing a factor.
However, in all 20 of the midterm years seen since 1946 - the S&P 500 has been higher 12 months after election day, with an average gain of 14.4%.
Despite the pullback in the early hours of Wednesday, the Nasdaq 100 remains up 12% in the year to date, with the S&P 500 advancing 10%.
Looking ahead, Seth Hickle of Mindset Wealth Management said: "I expect volatility to remain elevated over the next few weeks, regardless of the election outcome."
Freedom Capital Markets' chief market strategist Jay Woods told Sky's US partner CNBC: "Now all eyes turn to the president and the reaction that we may get from him.
"Will there be contested elections, will there be more uncertainty thrown into the ranks? That's where the market could really get riled up."
The attack, which affected three blood banks, sought to raise temperatures to extremely high levels but was thwarted before it could do any damage, authorities have said.
Temperatures in cold rooms were affected, but alarm systems at the sites meant that no blood stocks were damaged.
The incident comes after cyber attacks affected the UK meat processing industry and supermarkets in European countries reported that their fridges and freezers warmed up after they were remotely taken over - resulting in spoiled food costing millions of dollars.
The French Blood Establishment, or EFS, said in a statement that "temperature and defrost thresholds were raised to very high levels", adding that "no health consequences have been identified, and no blood products were lost".
EFS said the attack was not against itself, but on one of its providers.
In a statement to Sky News the EFS said: "The incident affected a limited number of EFS sites. Further detailed technical information from our service provider will enable us to accurately assess the full extent of the incident.
"The variations concerning the cold rooms system were detected by our monitoring and alarm systems rapidly, allowing our teams to take the necessary action to protect all stored blood products and samples. We are not specifically commenting on the actual temperatures.
"At this stage, we are not in a position to comment on the origin of the cyber incident or identify those responsible. The EFS teams are fully mobilised and working in coordination with the French National Cybersecurity Agency (ANSSI) and the Ministry of Health.
"No impact on the safety of blood products or samples has been identified. All products and samples stored in the affected equipment have either been secured or transferred as a precautionary measure to other facilities.
"In addition, monitoring of the operation and temperatures of all our refrigeration equipment has been strengthened. The continuity of our blood collection and distribution activities is fully ensured."
The products stored were bags of plasma intended for transfusions and plasma used to manufacture medicines, and were either transferred or secured, the EFS said in a statement.
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Speaking to Sky News, Andy Kay, chief operating officer of John Leisure, which supplies footballs and other leisure products to Poundland, said a probe of Gordon Brothers' behaviour during the retailer's sale process was necessary.
Poundland, which employs roughly 10.500 people, has received a solvent takeover bid from Northern Ireland-based Martin Property Group, which was rejected by Gordon Brothers because it failed to repay in full and immediately a £30m shareholder loan on completion of a deal.
On Wednesday morning, ITV News reported that Xyra Group Holdings (XGH), a little-known investment firm, had also tabled an offer which would keep Poundland out of administration.
Discussions are continuing with the bidders, although the likelihood of a deal being achieved without the company being put through an insolvency process diminishes with each week that passes.
Mr Kay demanded that Gordon Brothers "explains why a credible, fully-funded rescue of Poundland as a going concern cannot be achieved, particularly when the alternative could have devastating consequences for employees and suppliers".
That question is at the centre of escalating political interest in the crisis, with Poundland's bosses due to meet business minister Kate Dearden on Wednesday, and business secretary Jonathan Reynolds set to hold talks with Gordon Brothers executives later in the week.
MPs on the business select committee have also summoned Gordon Brothers to attend a hearing next week.
A notice of intention to appoint administrators, which was filed late last week, is due to expire next Thursday, although there is an option to extend it if a rescue deal remains a viable option.
"With Martin Property Group and now Xyra Group Holdings both reportedly seeking to acquire Poundland as a whole business, Gordon Brothers has some very serious questions to answer if neither transaction can be made to work," Mr Kay said.
"If two credible, properly funded buyers are prepared to rescue Poundland, Gordon Brothers must explain precisely what is preventing a solvent outcome and why breaking up the business would deliver a better result."
Sky News revealed on Tuesday that Gordon Brothers had written to the government to defend its 16-month stewardship of the retailer, claiming it had backed it with £85m of funding.
Mr Kay added: "One of the biggest unanswered questions is what happens to the millions of pounds of stock supplied to Poundland, who ultimately benefits from its value in an administration or break-up, and what losses suppliers are expected to absorb.
"The question isn't simply how many people Poundland employs, but whether anyone has properly calculated the economic damage that could follow its break-up.
"Suppliers have their own employees, financial commitments and businesses to protect.
"Those consequences deserve to be part of the decision, not an afterthought."
John Leisure is part of German-based John GmbH, which is marking its centenary this year.
Mr Kay stopped short of calling on ministers to rescue Poundland with taxpayers' money but urged them to "consider what assistance, if any, may be appropriate".
"Any decision involving taxpayers' money must be transparent, properly scrutinised and in the public interest," he added.
Gordon Brothers repeatedly failed to respond to messages seeking comment.
The 16-year-old from Southampton was arrested on suspicion of attempted murder.
An investigation was launched after five boys were stabbed just after 10.30pm.
Police said an uninvited group jumped a garden fence and seriously assaulted the victims - three who are aged 16 and two aged 17 - there and in surrounding streets.
Four of the boys suffered serious injuries and have now been discharged from hospital. The fifth, aged 17, remains in hospital in a life-threatening condition.
Officers have also been granted an additional 36 hours to question a 16-year-old and 17-year-old, both from Southampton.
The pair were arrested on suspicion of attempted murder on Monday morning.
Meanwhile, a 16-year-old boy from Bristol also remains in police custody, having been arrested on suspicion of attempted murder on Monday night.
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The Public Accounts Committee (PAC) has said a Notre-Dame-style fire or the King being plunged into darkness during the State Opening of Parliament were among the "real and rising risks" of continued postponements to the restoration and renewal programme.
Without action, the best case scenario would see the historic building crumble away into managed decline, the committee added.
In its latest report, published on Wednesday, the PAC estimated that the weekly cost of maintenance on the parliamentary estate is expected to grow from £1.5m to £2m in the coming years, warning that each year the programme is delayed will add £420m to the cost of delivering the scheme.
The report comes ahead of a series of votes in parliament to decide the future direction of the restoration programme, which were expected to take place earlier this year but have been delayed.
MPs first voted to leave parliament to allow the building to undergo extensive repairs in 2018, but there has been continued disagreement about whether this should proceed.
Sir Geoffrey Clifton-Brown, chair of the PAC, said that "each kick of the can down the road costs hundreds of millions in public money", and urged those looking to delay a decision further to "imagine the blow suffered by the UK's morale and reputation of some kind of catastrophic failure".
"A lack of action risks, for instance, the chamber being plunged into darkness during the State Opening of Parliament," Sir Geoffrey added.
"Consider the impact of global rolling news footage of Notre-Dame-like flames licking out of the building's roof, resulting in the destruction of our heritage, or worse, injuries or loss of life.
"These are not notional possibilities, but real and rising risks which our committee identified three years ago."
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Several options under consideration
Parliamentarians will be expected to narrow down the number of options for how the refurbishment programme will be carried out from four to two.
The first recommended option, a "full decant", could cost up to £15.6bn and last up to 24 years, while the second proposal, a partial decant, could last more than six decades and cost close to £40bn.
MPs and peers have also been asked to agree on the initial phase of the programme - capped at £3bn - which would focus on moving both Houses to temporary locations, as well as on relocating offices outside the palace.
The PAC, however, has warned that the temporary sites for MPs and peers may not be ready for when they would be required to leave the Palace of Westminster for restoration work to begin - which according to current timescales would be in 2032.
Sir Geoffrey also acknowledged that all options for the programme would "involve extremely high figures with very big ranges".
A parliament spokesperson said the PAC report "provides welcome scrutiny", adding: "Work is already happening across the parliamentary estate to ensure the ongoing safety of those working here and visiting."