Six weeks after a reluctant parliament finally granted Boris Johnson the general election he had demanded, Britain went to the polls on Thursday in a high-stakes vote widely described as the most important in a generation.

Following a campaign dominated by Brexit and the NHS, voters waited outside polling stations across the country to elect a new government for the third time in five years, with most pollsters viewing the result as highly unpredictable.

There had been fears that the low temperatures and widespread showers predicted for the first December polling day in almost a century would persuade many voters to stay indoors.

But early indications suggested turnout could be high, with constituents in multiple seats posting pictures of lengthy queues outside polling stations, particularly in London, and waits of more than half an hour to vote.

“The biggest queue I’ve ever seen at my polling station,” tweeted one south London voter, Kelly Molloy, at 8.13am alongside a picture showing dozens waiting to vote in her Streatham constituency.

Another south Londoner, Chris Schofield, said more than 70 voters were waiting in the Bermondsey and Old Southwark constituency – some of whom gave up and left during his 20-minute wait, “presumably to go to work”.

“It’s about 20 times busier than it was in 2017, and for the locals and Euro elections,” the 27-year-old consultant said. “Atmosphere is very London: orderly queueing and no one is talking to each other!”

Voters in the Labour marginal of Battersea and the Tory marginal of Putney shared similar photos, while queues were also reported in Cambridge and Manchester. In the Scottish National party target seat of East Lothian, campaigners also said voting levels had been unusually high, suggesting people had been eager to cast their vote during daylight hours.

Facebook Twitter Pinterest People queuing to vote in Balham, south London. Photograph: @onzlo99/PA

Wandsworth council, which covers Battersea, Putney and the safe Labour seat of Tooting, later confirmed that “unprecedented numbers” of people had turned out to vote early on Thursday.

Students also queued to vote at campus polling stations in the ultra-marginal constituency of Canterbury, where Labour is defending a lead of 187, in Exeter and in Lincoln, another Labour marginal.

In Bermondsey, south-east London, a burst water main caused deep flooding, leading some voters to decide to leave and return later. Graham Kings said: “I could have gone home and put wellington boots on and waded across the flooded road to try to get in, but had to go to work and so will vote this evening.”

Many members of the public said they were encouraged by the queues, suggesting it could mean a greater turnout than in the last general election. Total turnout at the 2017 general election was 68.8%, the fourth successive election where turnout increased.

Despite facing an energetic tactical voting campaign to oust him from his own constituency of Uxbridge, Boris Johnson was the first sitting prime minister in many years not to vote for himself, choosing instead to cast his vote close to Downing Street in the marginal constituency of Cities of London and Westminster.

The Tories, who hold the seat, are facing a high-profile challenge from Chuka Umunna, who left the Labour party earlier this year and is contesting the seat for the Liberal Democrats.

The former prime ministers Theresa May and David Cameron chose to vote in their home constituencies of Maidenhead and Witney in 2017 and 2015 respectively, while both Gordon Brown and Tony Blair would habitually start election day in their seats of Kirkcaldy and Cowdenbeath, and Sedgefield.

Johnson, by contrast, arrived at a polling station in Methodist Central Hall around 8am accompanied by his dog, Dilyn, kissing the animal for waiting photographers as he left.

Facebook Twitter Pinterest Boris Johnson outside the polling station in London with his dog, Dilyn. Photograph: Henry Nicholls/Reuters

The US Democratic representative Alexandria Ocasio-Cortez lent her support to Labour with a call for the UK public to vote.

Sharing an anti-Conservative video originally posted by Jeremy Corbyn, Ocasio-Cortez tweeted: “This video is about the UK, but it might as well have been produced about the United States.

“The hoarding of wealth by the few is coming at the cost of peoples’ lives. The only way we change is with a massive surge of *new* voters at the polls. UK, Vote!”

Corbyn was greeted by supporters and a lone protester as he and his wife, Laura Alvarez, arrived to cast their votes in his constituency of Islington North in London. Footage appeared to show the female protester, who was dressed as Elmo from Sesame Street, remonstrating with security guards and police who tried to block her, before Corbyn intervened to ask: “Hello guys, can we stop the arguing please?”

The Liberal Democrat leader, Jo Swinson cast her vote at a primary school in Bearsden in her constituency of East Dunbartonshire, while the Scottish first minister, Nicola Sturgeon, posed for photographers at a ballot box in her home constituency of Glasgow East.

Facebook Twitter Pinterest Jo Swinson and her husband, Duncan Hames, after voting in East Dunbartonshire. Photograph: Gonzalo Fuentes/Reuters

In Northern Ireland, tactical voting campaigns and electoral pacts mean a number of seats will be watched closely for potential upsets.

Arlene Foster, the leader of the Democratic Unionist party, wore a union jack scarf to cast her vote for a rival unionist in her home seat of Fermanagh and South Tyrone, after her party withdrew from the constituency in a bid to unseat Sinn Féin.

In North Belfast, her colleague Nigel Dodds, the leader of the DUP in Westminster, faces a threat from Sinn Féin’s John Finucane, after the Social Democratic and Labour party withdrew to help his campaign.

Speaking in Doncaster, the Brexit party leader, Nigel Farage, told reporters he was hoping for “very, very heavy rain” in the town on Thursday, in the hope it would depress the votes of the other parties.

Facebook Twitter Pinterest A man dressed as Father Christmas enters his grotto at the Dunster Tithe Barn near Minehead, Somerset, which is being used as a polling station. Photograph: Ben Birchall/PA

In Scotland, a 48-year-old man was arrested after explosives officers were called to investigate a suspicious device near a polling station in North Lanarkshire.

Residents were evacuated as a controlled explosion was carried out on the package at the Glen Tower flats in Motherwell, with constituents registered to vote at Glen Tower redirected to a nearby polling station. Police Scotland later said the item had been found to be a “non-viable device”.

Play Video 3:31 Election night 2019: what to watch for – video explainer

With the prime minister vowing that a Conservative victory would lead to Britain exiting the EU within weeks, Labour promising a second referendum and the Liberal Democrats offering to overturn the 2016 vote entirely, the campaign was inevitably dominated by the issue of Brexit, with the Conservatives seeking to boil down their campaign message to a bald promise to “get Brexit done”.

A cautious Tory campaign sought to limit access to Johnson, most notably in the prime minister’s refusal to take part in a keynote interview with the BBC’s Andrew Neil, unlike the other major party leaders.

Facebook Twitter Pinterest Early morning voters arrive at a mobile polling station at Holcombe village in the marginal Bury North constituency. Photograph: Christopher Thomond/The Guardian

The party’s manifesto was light on concrete promises, while the party was accused of hiding many leading Conservatives – notably Jacob Rees-Mogg and other hardline European Research Group members – from the limelight for much of the campaign.

Labour sought to keep the focus on the NHS, insisting the party would save the health service from Tory plans to privatise it. Its own manifesto made big-spending pledges to increase funds for the NHS, give free broadband to every home and compensate the so-called “Waspi” generation of women who lost out when pension rules were changed.

But the party has also struggled to shake off the issue of antisemitism during the campaign, an issue which John McDonnell, the shadow chancellor, acknowledged could influence the election result.

Polling throughout has consistently shown the Conservatives comfortably ahead, though the party’s lead has shrunk throughout the campaign.

The final poll, released on Thursday and conducted by Ipsos Mori for the London Evening Standard, gave the Conservatives an 11-point lead over Labour, but with one in four respondents saying they could change their minds, and pollsters having been proved wrong in 2015 and 2017, the result remains unpredictable.