Americans are voting in one of the most divisive presidential elections in decades, pitting incumbent Republican Donald Trump against his Democratic challenger Joe Biden.
The first polls opened from 05:00 EST (10:00 GMT) in Vermont.
Nearly 100 million people have already cast their ballots in early voting, putting the country on course for its highest turnout in a century.
Both rivals spent the final hours of the race rallying in key swing states.
National polls give a firm lead to Mr Biden, but it is a closer race in the states that could decide the outcome.
In the US election, voters decide state-level contests rather than an overall single national one.
Among the first swing states to begin election-day voting on Tuesday were the key battlegrounds of North Carolina and Ohio (11:30 GMT), followed half an hour later by Florida, Pennsylvania, Georgia, Michigan and Wisconsin. Arizona will follow at 13:00 GMT.
To be elected president, a candidate must win at least 270 votes in what is called the electoral college. Each US state gets a certain number of votes partly based on its population and there are a total of 538 up for grabs.
This system explains why it is possible for a candidate to win the most votes nationally – as Hillary Clinton did in 2016 – but still lose the election.
Control of the Senate is also at stake in these elections, with the Democrats seeking to gain control of both houses of Congress and the White House for the first time since early in Barack Obama’s first term.
The coronavirus pandemic has at times overshadowed the election campaign, with the epidemic in the US worsening over the final weeks of the race. The country has recorded more cases and more deaths than anywhere else in the world, and fear of infection has contributed to an unprecedented surge in early and postal voting.
There are fears that pockets of post-election violence could break out as the results come in.
A new “non-scalable” fence has been put up around the White House in Washington DC. Businesses in the nation’s capital and also in New York City have been seen boarding up their premises due to concerns about unrest.
On Monday, President Trump, 74, who while behind in the polls could still carve out a victory, sprinted through four more battleground states.
With stops in North Carolina, Pennsylvania, Michigan and Wisconsin, he talked up his economic record and prospects for victory, while talking down the polls and the media, and mocking his opponent.
“We are going to gain four more years in this very beautiful White House,” he told supporters at his final rally in Grand Rapids, Michigan, the same industrial city where he ended the 2016 election race.