Washington: US President Donald Trump on Sunday repeated his allegations of electoral fraud by claiming without evidence that the voting machines were "corrupt" and termed it a "stolen election".

Trump has repeatedly attempted to question the validity of vote counts in key swing states and made unfounded claims that the election is being "stolen" from him.

"We believe these people are thieves. The big city machines are corrupt. This was a stolen election. Best pollster in Britain wrote this morning that this clearly was a stolen election, that it's impossible to imagine that Biden outran Obama in some of these states, Trump said in a series of tweets on Sunday morning.

A day earlier, the mainstream media projected his Democratic challenger Joe Biden as the winner of the November 3 presidential election.

States are yet to certify the votes. In the American presidential electoral system, it is the media which traditionally calls on the election, which is followed by a formal official announcement of the results. The media's call is based on the voting trends and past history.

Unlike in the past, Trump has not conceded the election so far, asserting that official certified vote count has not been announced yet.

The Trump campaign, which has filed a number of lawsuits and is planning to slap many others across the country, is alleging voter fraud and electoral malpractice.

"Where it mattered, they stole what they had to steal, Trump said in another tweet.

All of his tweets were flagged by Twitter on Sunday, noting that these were disputed claims by the president. For the first time ever in the history of Twitter, the social media giant has been flagging and censoring so many tweets of the head of a state.

"We should look at the votes. We're just beginning the tabulation stage. We should look at these allegations. We're seeing a number of affidavits that there has been voter fraud. We have a history in this country of election problems," Trump alleged.

"In Pennsylvania you had an order by a Supreme Court Justice to compel them to separate ballots that were received after the legislative deadline. It required the intervention of Justice Alito. That's a large group of ballots," he said.

"When you talk about systemic problems, it's about ..how these ballots were authenticated, because if there's a problem in the system about authentication, that would seriously affect the ENTIRE ELECTION - And what concerns me is that we had over a hundred million mail-in ballot in cities like Philadelphia and Detroit with a long series of election problems (to put it mildly)," Trump claimed.

Trump drove down from the White House to his golf course in a Virginia suburb to play golf, before sending out the tweets.

His campaign has now established a "Election Defense Fund" to raise funds to fight the expensive legal battle in US courts that might end up in the US Supreme Court.

"Only YOU can save America from the Radical Left. Don't let your Country down, Trump said in one of his mass fundraising emails.

"Democrats have made it clear they'd rather destroy our Nation than have four more years of our President's incredible leadership," Vice President Mike Pence said in another email.

Trump on Saturday claiming without evidence that "bad things happened" inside the counting rooms and tens of thousands of votes were illegally received in the key battleground state of Pennsylvania.

Trump at a White House news conference on Friday accused the Democrats of massive ballot fraud and alleged that the election was being stolen. He did not offer any evidence to back his claim.

The Biden campaign has denied the allegations.