In a speech from the White House, Mr Trump said he was set to win Arizona as postal votes came in, but accused Democrats, without evidence, of stealing an election.

"I challenge Joe and every Democrat to verify they are legal votes because they talk about votes and they should use the word legal votes," he said.

"Our goal is to defend the integrity of the election and we will not allow the opposition to steal the election."

No candidate has been declared the victor in the election with neither having reached the 270 electoral votes required to win.

Mr Trump also hit out at polls, with false claims they had been designed to keep Republican voters at home.

It was a stark contrast to Democratic candidate Joe Biden, who called for calm as vote counting continues in several key battleground states.

In an address to the media from his home state of Delaware on Friday morning (AEDT), Mr Biden said every vote must be counted.

"In America, the vote is sacred, and it's how people in this nation express their will," he said.

"It is the will of the voters, no one, not anything else, that chooses the president of the United States of America.

"Democracy is sometimes messy, and sometimes it requires a little patience as well. But that patience has been rewarded now for more than 240 years with a system of governance that has been the envy of the world."

Mr Biden said he and his running mate, Senator Kamala Harris, were confident of victory in the presidential race.

It comes as vote counting stretched into a third consecutive day in several key battleground states.

Several states including Pennsylvania, Georgia, Arizona, Nevada and North Carolina still remain too close to call, but the gaps between candidates have narrowed as officials count early votes and votes sent in by mail.

According to The New York Times, Biden is ahead on 253 electoral votes compared to Trump's 214.

Election officials in Pennsylvania overnight said the "overwhelming majority" of the state's 550,000 outstanding votes should be tallied later on Friday (AEDT) and a winner could be known by then.

Biden is behind in the state by a little more than 60,000 votes, however, he has narrowed a gap of more than 700,000 votes in Pennsylvania as more mail-in votes come through.

Votes that are yet to be counted are in metropolitan areas that are more likely to support Joe Biden than his opponent.

If Biden claims a victory in Pennsylvania, he will have reached enough electoral votes to win the presidency.

Meanwhile in Georgia, Trump is leading by just 3000 votes after the Biden camp narrowed the gap from more than 200,000 votes.

A Trump victory is still possible, but his path to winning the White House has narrowed.

On Twitter, Biden said he remained confident about a possible win and urged for calm from his supporters.

"Be patient, folks. Votes are being counted, and we feel good about where we are," he said.

Protesters who support Donald Trump have called for the vote count to be stopped in Pennsylvania due to the closing gap between the two candidates.

Large protests were seen in parts of Philadelphia overnight.

In a statement issued by the President, he falsely claimed that votes being counted after election day were illegal and urged the count to be stopped.

Meanwhile, several lawsuits launched by the Trump campaign in Michigan and Georgia to either force a recount of the results or stop the vote count were dismissed by state officials.

However, a lawsuit filed by Trump's campaign in Pennsylvania to allow election observers better access to vote counting areas succeeded.

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While Trump supporters are calling for the vote count to be halted in Pennsylvania due to the narrowing result, his supporters in Arizona have protested calling for more votes to be counted.

While Biden recorded an early lead in the state, Trump has narrowed the gap to now being behind by a little more than 68,000 votes with 14 per cent left to be counted.

Arizona has only been won by the Democrats twice since 1948.

The race between the two candidates is also narrowing in Nevada with Biden ahead by around 10,000 votes with 11 per cent of the votes left to be counted.