Donald Trump accused Democrats of an attempt to commit fraud as he trailed challenger Joe Biden in early results.
The US president provided no evidence to back his allégations.
Speaking from the White House, Trump vowed to seek a court order to halt vote counting.
"This is a major fraud on our election. We want the law to be used in a proper manner. So we'll be going to the US Supreme court. We want all voting (he means counting) to stop. We want them to find any ballots at 4 o clock in the morning and add them to the list," Trump said.
As of 09:00GMT, Trump had garnered 213 electoral votes against Biden's 238. 270 votes are needed for one to be declared the winner.
Joe Biden was the first to speak. He told supporters from Delaware that he was confident of a Victory.
"We're feeling really good about Wisconsin and Michigan. And by the way, it's going to take time to count the votes. We're going to win Pennsylvania," said the 77-year-old.
Trump won the state of Florida, a big prize with 29 electoral votes, denying Biden an early win. Arizona was called for Biden by AP, the first state to flip from Red to Blue in this election, a massive boost for the Democrats.
Police are deployed in several cities as fears of a possible explosion of clashes grew after the tense vote.