Live Updates: U.K. Votes in General Election

Britons were trudging through the doors of community centers, churches, pubs and former miners’ clubs to cast ballots in a pivotal election underlined by the country’s 2016 referendum on leaving the European Union.

It is the third general election since 2015 and the first to be held in December in nearly 100 years. Voters are choosing representatives for their local districts in Parliament: 650 lawmakers in all for the House of Commons, which decides the country’s laws and policies and chooses the prime minister.

While Brexit has dominated the campaign — with Prime Minister Boris Johnson of the Conservative Party putting the issue at the center of its campaign and vowing to “get Brexit done” — other major issues may determine the outcome. The opposition Labour Party, led by Jeremy Corbyn, has put health care at the center of its agenda and has framed itself as the defender of the National Health Service.

Smaller parties — including the Liberal Democrats, Greens, Brexit Party and pro-independence parties in Scotland and Wales — could also play a decisive role.

Polls are open until 10 p.m., but according to the Electoral Commission, any eligible voters in line inside or outside polling stations at 10 must be allowed to cast a ballot.

The results of exit polls will begin to emerge almost immediately. Official results are expected overnight: Friday, Dec. 13.

Voters across London described crowded polling stations, posting photographs of the long lines. Few said that their ability to vote was affected, but many noted that such delays were unusual in Britain.

One voter, Ed O’Meara, shared pictures from his polling station, in the Balham and Tooting area of South London, that showed a line stretching out the door and up the road.

A voter at a different polling site in Balham described a 20-minute wait. Those casting ballots outside the capital described similar scenes.

“That’s the first time I’ve ever had to queue & wait outside of a polling station in order to exercise my right,” wrote one Twitter user identified as John E. Walsh, who shared a photograph said to be taken in Cambridge.

The University of Essex also shared a video of dozens of students waiting to vote, calling the turnout “fantastic.”

Once polls opened, British broadcasters and news websites had to pivot. It’s illegal for anyone in the country to publish information on how citizens say they have voted — exit polls or forecasts — until after polls close at 10 p.m.

A code of conduct laid out by Britain’s communications regulator, Ofcom, specifies that all discussion and analysis of election issues on television and radio must cease once polls open, that no opinion polls can be published and that no coverage of opinion polls is allowed while people are voting.

“When people are going to the polls on Election Day, it’s important that everyone can vote on the same information,” the regulator said.

The Guardian’s live briefing urged readers to comment, but to avoid saying how they voted.

“There will be no coverage of any issues directly pertinent to the election campaigns on any BBC outlet,” according to the public broadcaster’s internal election guidelines.

But even before that, the broadcaster found itself in hot water on Wednesday night, when Laura Kuenssberg, a political editor, offered a short assessment of the postal vote. (Party candidates and others can observe postal votes being verified, but the ballots are not counted until polls close on Election Day.)

The BBC denied that her comment broke the law.

#Dogsatpollingstations has become something of an Election Day tradition in Britain, with voters sharing photographs of their pups outside their local polling stations.

Several high-profile voters got in on the action on Thursday, with Sadiq Khan, the mayor of London, accompanied by his dog, Luna, and Prime Minister Boris Johnson arriving with his dog, Dilyn.

In much of Britain, the dogs and their owners had to brave a cold, wet morning at the polls, but few seemed to mind.

In the run-up to the election, manipulated Twitter accounts, doctored videos and dodgy websites became part of everyday life in Britain.

When an accurate story about a young boy being forced to lie on the floor in an overcrowded hospital quickly became an election issue, disinformation was at the fore in the form of a social media campaign to discredit the boy’s family.

While questions have been raised about foreign meddling and international disinformation campaigns, a surprising amount of questionable behavior and content has come from the political parties and candidates themselves.

The use of disinformation techniques by political leaders, particularly Prime Minister Boris Johnson’s the Conservative Party, points to an evolution in how the internet is being used to grab attention, distract the news media, stoke outrage and rally support.

“This is the election where disinformation was normalized,” said Jacob Davey, a senior researcher at the Institute for Strategic Dialogue, a London-based group that tracks global disinformation campaigns.

“A few years ago people were looking for a massive coordinated campaign from a hostile state actor. Now, many more actors are getting involved.”

The Labour Party was once a natural fit for Jewish voters, but with accusations of anti-Semitism rife, many are left feeling stuck with a choice between lesser evils in the election.

Jewish voters in Britain overwhelmingly opposed Brexit, the issue at the core of Prime Minister Boris Johnson’s Conservative Party campaign, dreading a resurgent far right and the splintering of the European Union.

But they are also reluctant to hand power to Jeremy Corbyn and his Labour Party after an avalanche of anti-Semitism accusations against the party.

“I feel quite torn,” said Keith Kahn-Harris, a Jewish sociologist and writer. “The issue in the Jewish community at the moment is anti-Semitism is something you can’t hold your nose for, the one thing you can’t overlook, which I understand. But for me, there are multiple things I can’t overlook, and it’s very difficult to know how to balance them.”

The anti-Semitism scandal is “gravely damaging Labour’s reputation as the nice party,” said Glen O’Hara, a historian at Oxford Brookes University.

The youths of Britain may again help steer a major shift in the election’s outcome, as it did in the 2017 vote.

Then, a large youth turnout and a dominant performance among young voters helped Labour win enough seats to unexpectedly deprive the governing Conservative Party of its parliamentary majority.

Brexit has been viewed by many young people as a threat to their ability to travel, study or work abroad. Along with the environment, education and housing, it continues to be one of the most important issues for young voters, according to an analysis by YouGov, an internet market research firm based in Britain.

“This election is the most significant of our time. It will determine our future,” said Harriet Farmer, 19, a student at the University of Southampton. “Young people are always overlooked, but in this election, we will make ourselves heard. We are engaged, we have registered and we will vote our way out of this mess.”

As the campaign played out in Britain, a Times reporter, Patrick Kingsley, spent two weeks driving from London to Glasgow, speaking with residents about the state of the nation at this critical moment.

During the 900-mile journey through Britain last month, he tried to make sense of a splintered country in the run-up to the general election.

“Everywhere I went, it felt as if the country were coming unbound,” he wrote. “For all sorts of reasons, all sorts of people — Leavers and Remainers; blue- and white-collar; Jews and Muslims; English, Northern Irish, Scottish and Welsh — felt alienated and unmoored.”

In the six weeks leading up to the vote, candidates, particularly women, found themselves in a climate where they say abuse, threats and a culture of intimidation have become the norm.

Rachel Reeves, the Labour candidate who has represented West Leeds in Parliament since 2010, described canvassing precautions and new security measures in her office that became necessary in this election campaign.

“I do think it’s a very different atmosphere and environment now,” Ms. Reeves said, compared with the previous two elections in which she ran. “People are a lot angrier and there’s a lot more polarization, particularly around the Brexit issue.”

And the issue affects candidates across the political spectrum, with women in all of the major parties citing similar experiences.

While men have seen a fair share of abuse, as well, more than a dozen female lawmakers cited threats and online abuse as part of their rationale for not running in the election, again bringing the issue to the fore.

Prime Minister Boris Johnson’s Conservatives have to win about 320 seats in Thursday’s general election to gain a mandate to govern. If they win a majority, Britain would be on course to leave the European Union on Jan. 31 and then begin trade talks with the bloc.

Jeremy Corbyn’s Labour Party has a tougher hill to climb to victory, needing about 80 seats to gain a majority. Polls give Labour daunting odds, but if it manages to form a minority government, a second Brexit referendum would be in the cards.

But what happens to Brexit in the case of a result that falls short of either outcome? The answer is that it would prove messy — and could even mean the country crashing out of the bloc without any agreement next month.

Were Mr. Johnson to win around 310 seats — perhaps 10 or so short of an outright majority — he would almost certainly try to stay in power, said Joe Greenwood, a politics fellow at the London School of Economics. But he would struggle to pass the legislation required to implement his withdrawal agreement with the European Union.

In theory, Mr. Johnson could request another Brexit extension, or promise a second referendum to break the deadlock — but that would leave him open to a challenge from his own strongly pro-Brexit lawmakers.

In terms of getting his deal through, he would lack potential allies among smaller parties, having alienated the Democratic Unionist Party, the ultraconservative grouping from Northern Ireland that propped up the Tories before the election.

Unless they could be won over, Mr. Johnson may have to rely on the support of a few opposition Labour lawmakers to try to squeeze his Brexit plan through — with the alternative being a disruptive “no-deal” exit.

That could make for more frenetic scenes in Parliament in January.

Reporting was contributed by Mark Landler, Stephen Castle, Amie Tsang, Megan Specia, Adam Satariano, Benjamin Mueller and Patrick Kingsley.