Most polls have closed in the key battleground states of Florida, North Carolina, Ohio and Georgia, which could offer early signs into the race for the White House between President Donald Trump and former vice president Joe Biden.
Results from Florida could come in faster than most states as it has had a weeks-long head start on counting mail-in ballots.
Ohio is also likely to report early voting and mail-in ballots quickly.
But the North Carolina State Board of Elections said that results would be delayed because it is keeping four polling locations open because of a late start this morning.
Mr Trump is expected to remain at the White House as early results come in, while Mr Biden was scheduled to make an address from his home town of Wilmington, Delaware.
Unless either candidate demonstrates a strong early showing in any of the eastern battleground states, a clear winner may not be announced for several days or even weeks.
Mr Biden has so far won Virginia, Rhode Island, New Jersey, Massachusetts, Maryland, Illinois, Delaware, Connecticut, Vermont, New York and New Mexico.
Donald Trump has won South Carolina, Kentucky, Oklahoma, Tennessee, Mississippi, Alabama, West Virginia, Arkansas, Indiana, North Dakota, South Dakota, Wyoming, Louisiana and Nebraska.
None of the called states so far is a swing state.
Although the timing of the results is uncertain, with many states expected to continue counting ballots over the coming days, Mr Trump has repeatedly insisted, without any legal basis, that states announce a clear winner on election night.
Businesses throughout Washington and other major cities in the deeply divided country were boarded up in case of civil unrest.
A US federal judge ordered a sweep of mail processing plants in 12 postal districts for undelivered ballots in Florida, Michigan, Pennsylvania and Alabama.
The FBI announced an investigation into automated phone calls in Michigan and other states that tried to mislead voters into delaying their votes until tomorrow.
The FBI has issued a warning about potentially violent election-related clashes in Portland, Oregon.
And the Department of Homeland Security urged vigilance against potential foreign interference in the election, urging Americans “to treat all sensational and unverified claims with scepticism, and remember technology sometimes fails”.
More than 100 million people cast their ballots before election day, about 73 per cent of almost 139 million ballots cast in 2016.
A Morning Consult poll found that 93 per cent of Biden voters said controlling the spread of Covid-19 was their top issue, while 83 per cent of Trump voters said protecting the US from terrorism is their biggest priority.
North Carolina and Georgia also have two of the most competitive Senate races in the country.
Republican incumbent Thom Tillis is facing stiff competition from Democratic challenger Cal Cunningham in North Carolina.
Meanwhile, Georgia has two separate Senate races – Republican incumbent David Perdue running neck and neck with Democrat Jon Ossoff in the first.
The other Republican incumbent, Kelly Loeffler, is vying for a place in the state’s run-off race with fellow Republican Doug Collins and Democrat Raphael Warnock.
Democrats are widely expected to maintain power in the House of Representatives and hope to take control of the Senate from the Republicans.
In Kentucky, Senate Majority Leader Mitch McConnell fended off Democrat Amy McGrath, but he acknowledged his GOP colleagues face tougher races.
