A jaded electorate is delivering its verdict on Prime Minister Rishi Sunak's Conservative Party, which has been in power since 2010.
Polls opened at 40,000 stations, including churches, a laundromat and a crematorium.
"Nothing has gone well in the last 14 years," London voter James Erskine said, adding he was optimistic for change.
"I just see this as the potential for a seismic shift, and that's what I'm hoping for."
Prime Minister Rishi Sunak's Conservative Party has been in power in the UK for 14 years. (EPA PHOTO)
Hundreds of communities were locked in tight contests in which traditional party loyalties come second to more immediate concerns about the economy, crumbling infrastructure and the National Health Service.
In Henley-on-Thames, about 65km west of London, voters like Patricia Mulcahy, who is retired, sensed the country was looking for something different.
The community, which normally votes Conservative, may change its stripes this time.
"The younger generation are far more interested in change," Mulcahy said.
"So, I think whatever happens in Henley, in the country, there will be a big shift. But whoever gets in, they've got a heck of a job ahead of them. It's not going to be easy."
I've changed the Labour Party. — Keir Starmer (@Keir_Starmer) If you put your trust in me by voting Labour, I will change the country.July 4, 2024
The UK has experienced a run of turbulent years - some of it of the Conservatives' own making and some of it not - that has left many voters pessimistic about their country's future.
The UK's exit from the European Union followed by the COVID-19 pandemic and Russia's invasion of Ukraine battered the economy, while lockdown-breaching parties held by then-prime minister Boris Johnson and his staff caused widespread anger.
Johnson's successor, Liz Truss, rocked the economy further with a package of drastic tax cuts and lasted just 49 days in office.
Rising poverty and cuts to state services have led to gripes about "broken Britain".
The first part of the day was sunny in much of the country - favourable weather to get people to the polls.
If Labour are handed a blank cheque, they will use their supermajority to tax you more for the rest of your life.— Rishi Sunak (@RishiSunak) July 4, 2024
In the first hour polls were open, Sunak made the short journey from his home to vote at Kirby Sigston Village Hall in his Richmond constituency in northern England.
He arrived with his wife Akshata Murty and walked hand-in-hand into the village hall, which is surrounded by rolling fields.
The Labour Party led by Keir Starmer has had a steady and significant lead in opinion polls for months but its leaders have warned against taking the election result for granted, worried their supporters will stay home.
"Change. Today, you can vote for it," he wrote on Thursday on the X social media platform.
A couple of hours after posting that message, Starmer walked hand-in-hand with his wife Victoria into a polling place in the Kentish Town section of London to cast his vote.
He left through a back door out of sight of a crowd of residents and journalists who had gathered.
Labour has not set pulses racing with its pledges to get the sluggish economy growing, invest in infrastructure and make the UK a "clean energy superpower".
But nothing has really gone wrong in its campaign either.
The party has won the support of large chunks of the business community and endorsements from traditionally conservative newspapers, including the Rupert Murdoch-owned Sun tabloid, which praised Starmer for "dragging his party back to the centre ground of British politics".
The Conservatives have acknowledged that Labour appears headed for victory.
In a message to voters on Wednesday, Sunak said that "if the polls are to be believed, the country could wake up tomorrow to a Labour super-majority ready to wield their unchecked power".
He urged voters to back the Conservatives to limit Labour's power.
But for many voters, the lack of trust applies not just to the governing party but to politicians in general
Veteran anti-establishment figure Nigel Farage has leaped into that breach with his Reform UK party, grabbing attention with calls to curb immigration.
The centrist Liberal Democrats and environmentalist Green Party also want to sweep up disaffected voters.
