The first indication of the outcome will come just after polls close at 10 p.m. local time (5 p.m. Eastern), when the major British broadcasters reveal the national exit poll. It’s a survey of thousands of voters just after they have cast their ballots, and has come close to the final result in recent elections, though there’s always a chance of that streak being broken, too.

The votes are counted overnight. A first couple of parliamentary districts usually finish their work within two hours of polls closing, and almost every district is expected to declare a winner by 7 a.m. local time (2 a.m. Eastern). Prime Minister Rishi Sunak and the Labour leader who hopes to replace him, Keir Starmer, would customarily speak when the results are declared in their own districts, probably after 2:30 a.m. local time for Mr. Starmer and after 4 a.m. local time for Mr. Sunak.

What happens next?

Should there be a clear opposition victory, the transition of power would take place with ruthless speed.

“If the election results in an overall majority for a different party,” says the Cabinet Manual, which sets out the official guidance on the process, “the incumbent prime minister and government will immediately resign and the sovereign will invite the leader of the party that has won the election to form a government.”

“Immediately,” in practice, would mean Friday morning.

By “recent custom,” according to the House of Commons library, departing prime ministers pose with their families for a final set of photographs in Downing Street, their home and workplace while in office.

There might be a last speech. “When the curtain falls, it’s time to get off the stage,” John Major, the last prime minister to give way to an opposition majority, said in 1997. “And that’s what I propose to do.”

Then comes a short drive to Buckingham Palace, usually trailed by news helicopters, to resign in a private meeting with the monarch, now King Charles III.

The next prime minister would be close behind: In 2016, according to the Commons library, the car of the incoming leader, Theresa May, arrived at the palace 32 seconds after her predecessor, David Cameron, had left.

A new leader’s appointment would also take the form of a private meeting with the king, usually right after the resignation. It’s known as “kissing hands,” though it involves little ceremony and no kissing.

Expect a photographed handshake, followed by another speech in Downing Street, where the new prime minister would move in straight away, applauded by the permanent civil service staff on arrival.

The prime minister would then appoint other ministers. It’s not usually a matter with much suspense: British oppositions maintain a “shadow cabinet” of candidates for government positions.

The new Parliament would meet for the first time in the next couple of weeks.

All this, of course, assumes a change of prime minister. If Mr. Sunak’s government unexpectedly maintains its majority, there’s no ceremony — he would simply continue in office.

What if no one wins a majority?

If no party were to win a majority of parliamentary seats, Mr. Sunak would stay on as a caretaker while parties negotiated with one another to decide who could govern.

It might not be a long delay, however: It took five days to reach an agreement in 2010, when Mr. Cameron fell short of a majority, and a couple of weeks in 2017, when Mrs. May did. And then, if the deal put someone else in charge, the cars would set off for the palace.

Older couples walked hand in hand into the local church, which had been temporarily fitted out with ballot boxes, alongside parents with children in strollers, and young adults rushing in on the way to work.

One by one, they weighed in on the future of the nation in a vote that polls suggested could end 14 years of Conservative-led government.

“I just want to see change,” said Sam Argha, 36, who was outside the polling station on Thursday morning. “I just really want to see us do something differently.”

Many people in the city expressed a similar desire for a new start at a moment of intense national uncertainty. Polls have predicted that the election could be a major turning point, with the center-left Labour Party expected to unseat the right-wing Conservative Party, possibly with a crushing landslide.

Portsmouth North is considered a bellwether seat — the area has voted for the winning political party in every general election since 1974.

It also serves as a microcosm of the broader national challenge facing the governing party: a longstanding Conservative constituency held by a popular candidate that is now at risk of being lost, and a largely disillusioned electorate that expressed frustrations with their quality of life and what many see as a lack of leadership.

The seat has been held since 2010 by Penny Mordaunt, a Conservative lawmaker whose prominent role at the coronation of King Charles III last year, when she wielded a heavy, jewel encrusted ceremonial sword, drew international attention to her steadiness and poise.

Ms. Mordaunt, who is seen as a possible contender for her party’s leadership, is widely liked in Portsmouth, and some locals said they had no intention of heading in a new direction. But polls have suggested that Labour voters in the constituency could still overtake Conservative support in Thursday’s vote.

The centrist Liberal Democrats — considered the third most popular party here — and the hard-right Reform U.K. party could also siphon off votes from the Conservatives.

“My hopes are for a much more compassionate government from Friday,” said Grahame Milner, 62, who was walking in the city center with his husband of three decades on Wednesday afternoon.

Many of the shops surrounding the couple were vacant or boarded up. Graffiti marked the sides of shuttered department stores. There is little to attract people to the area, other than the bookmakers, charity shops and small stores selling vapes, Mr. Milner said.

He first came here to serve in the Navy — the city is home to the country’s biggest naval base — and was deployed during the 1980s Falklands War as a chef aboard a military vessel. He was pushed out of the military because of his sexual orientation, he said, and later became deeply involved in union work after returning to civilian life. He had already cast his ballot by postal vote last week.

“The austerity program has been absolutely crippling to working-class people,” Mr. Milner said, pointing to the number of working people relying on food banks just to get by. “This is just not the Britain that I served in the military for.”

Concerns about the hollowing-out of the National Health Service, a cost-of-living crisis that has left many struggling, debates about immigration, and the fallout from Britain’s withdrawal from Europe were front of mind for many locals.

Some said they had no plans to vote at all, disillusioned by politicians from across the spectrum.

“It’s always been Labour for us, but I am not voting this year,” said Tracy Patton, 59, who has lived in the city all her life and said she was fed up with politics. She sat outside a cafe on Wednesday evening, reminiscing with friends about how the once busy marketplace had changed.

“It was bustling, there was atmosphere,” she said. “But now, it’s going through decline. There is just no money in England anymore.”

For some younger voters, the prospect of an uncertain future has weighed heavy. Daisy Quelch, 28, and Kiran Kaur, 24, were packing up after an outdoor boxing class near the waterfront on Southsea Common.

“Sometimes it feels like our world is crumbling,” Ms. Quelch said, adding that she was particularly worried about climate change and the environment. “We want to see changes, but it can’t happen quickly enough.”

Earlier this year, residents were warned not to swim in the sea as the local water company had released raw sewage along the coastline, contaminating the water.

Water pollution has become a campaign issue in many parts of Britain, as some blame the government for its inability to stop the water industry — which was privatized during the Conservative government of Margaret Thatcher in the 1980s — from releasing untreated waste into the waterways.

Some formerly stalwart Conservative supporters said they were rethinking their vote. Several were considering casting their ballots for Reform U.K., the populist anti-immigration party led by Nigel Farage, a brash and polarizing figure who has shaken up the general election campaign.

But others defended the Conservative Party. In Dixie’s Pub just off the high street, a group of patrons played pool on the eve of the election, the clack of billiard balls mingling with chatter.

Andrew Revis, 57, was enjoying a pint at the bar after finishing work at his nearby accountancy office, and said he felt that the Conservatives and Ms. Mordaunt, who he described as a capable and committed lawmaker, were receiving undue criticism.

“They are getting a lot of stick, but I don’t think it’s entirely been in their control,” he said, pointing to the devastating impact of the coronavirus pandemic and the war in Ukraine that created unexpected hardship.

“It’s the cost of living,” said Kerry Harris, 36, who sat outside the Iceland supermarket with her niece Shanice Bakes, 19, on Wednesday evening. She gestured to their bags. There was a time, Ms. Harris said, when a full shopping cart of groceries would cost about 50 pounds, or $65, but now she couldn’t fill one bag for that price.

“And they don’t put your wages up, do they?” she added.