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Facebook removed a rapidly-growing group that was spreading misinformation about the US presidential election, saying the group was causing real-world protests.

"In line with the exceptional measures that we are taking during this period of heightened tension, we have removed the group 'Stop the Steal' which was creating real-world events," Facebook said in a statement reported by US public radio.

"The group was organized around the delegitimization of the election process, and we saw worrying calls for violence from members of the group," Facebook added.

The group amassed over 300,000 members in a matter of days as it spread accusations that Democrats were seeking to undermine Republican votes to secure an election victory for their nominee, Joe Biden.

Election disinformation has rapidly spread online as the nation awaits results in a handful of key swing states that will decide the outcome.

"This is the most intense online disinformation event in US history and the pace of what we have found has only accelerated since Election Day," said Alex Stamos, a Stanford University researcher who tracks disinformation.

As anticipated, the flood of mail-in ballots during the pandemic has taken longer to count and largely favoured Biden, since Trump encouraged his supporters to vote in person.

This has created a situation where Trump's leads in some states have narrowed from election night as mail-in ballots are counted, leading to false accusations that the vote is rigged against Trump.

Excluding retweets, Trump has had nine tweets flagged or restricted by Twitter as he seeks to rally his base on the fraud allegations.

Trump's family members and prominent supporters are also having many of their tweets restricted.

The message from Twitter on such posts reads: "Some or all of the content shared in this Tweet is disputed and might be misleading about an election or other civic process."

Trump's Twitter rampage, and false accusations that the election is being stolen from him, have resonated with many of his supporters who have staged protests in Arizona vote centres while misinformation spreads.

A smaller group of protesters on Wednesday also appeared at a vote counting centre in the Democratic stronghold of Detroit, Michigan, shouting, "Stop the count!"

Meanwhile anti-Trump protesters have clashed with police in New York and in Portland, Oregon, as they call for all votes to be counted.

Trump's path to the White House is narrow after Biden was projected to win Wisconsin and Michigan on Wednesday, two key battleground states that Trump captured in the 2016 election.

Biden only needs to win one of the remaining states, if the former vice president holds his lead in Arizona, while Trump needs to win nearly every remaining state.