Prime Minister Theresa May called the snap election in the hope of increasing her Conservative Party's majority in Parliament, strengthening her hand in divorce talks with the European Union.
She had been expected to win comfortably, but a faltering campaign has reduced her poll lead over the left-wing Labour Party.
Polling stations close at 10 p.m. local time (5 p.m. ET) as voters choose 650 lawmakers for the House of Commons.
May says she will build a "stronger, fairer and more prosperous Britain," while Labour leader Jeremy Corbyn says he would govern "for the many, not the few."
In Scotland, the pro-independence Scottish National Party was aiming to defend its dominant position from recent gains by Conservatives.