US President Donald Trump and Democratic challenger Joe Biden will hold dueling prime-time town halls instead of a second presidential debate on Thursday, as Americans continued to swamp polling places in states allowing early in-person voting.

With less than three weeks to go until Nov 3's Election Day, the Republican president is trying to change the dynamics of a race in which Biden has a double-digit advantage in some national polls.

North Carolina, a highly competitive state, began more than two weeks of in-person early voting on Thursday, following huge turnout in Georgia and Texas earlier in the week.

Video from local media showed large numbers of people waiting for the polls to open in Greensboro and Winston-Salem, and gathering in the pre-dawn hours to vote at two arenas in the state's largest city, Charlotte.

Gerry Cohen, a member of the election board in the county that includes most of the city of Raleigh, saw more than 400 people in line at a community center before polls opened.

"I've never seen this many in line here," he said on Twitter.

Nearly 18 million Americans have cast ballots either in person or by mail so far, representing 12.9 percent of the total votes counted in the 2016 general election, according to the US Elections Project at the University of Florida.

Voters are seeking to avoid in-person lines on Election Day to stay safe as coronavirus infections and hospitalizations continue to rise, but also to make sure their ballots will count. Many are concerned that Trump will challenge widely used mail-in ballots, after he claimed without evidence that they were fraudulent.

Trump's campaign is counting on a surge of last-minute votes. But Reuters/Ipsos polling conducted from Friday to Tuesday shows there are far fewer undecided likely voters this year - around 8 percent - and they are just as likely to pick Biden as they are Trump.

Four years ago at this stage of the campaign, more than twice as many people were similarly wavering between Trump and his Democratic opponent, Hillary Clinton.

The Reuters/Ipsos polling shows Biden holding a 10-percentage-point lead nationally, with a tighter margin in the battleground states that will help decide the election.

Democratic fundraising organization ActBlue said on Thursday it collected $1.5 billion online from July to September, the most it had ever raised in one quarter. By comparison, major Republican fundraising platform WinRed said on Monday that it collected $623.5 million in the same period.

"We've raised more money than I ever thought we could," Biden told donors at an event.

Both candidates have been visiting battleground states this week, with Trump holding rallies in Florida, Pennsylvania and Iowa and Biden traveling to Ohio and Florida.

Speaking to a rally in Greenville, North Carolina, on Thursday, Trump promised an economic recovery if he was re-elected. "We're going to have a red wave," he said.

The US economy tanked in the second quarter due to the coronavirus pandemic, and at least 25 million people remained on jobless benefits at the end of September, Labor Department figures showed on Thursday.

Trump pulled out of Thursday's scheduled debate when the commission in charge of organizing the event said it would be held virtually after he contracted the coronavirus. A final debate is still scheduled for Oct 22 in Nashville, Tennessee.

The town halls, in which each candidate will field questions from voters, will take place at 8 pm EDT (0000 GMT), with Trump on NBC from Miami and Biden on ABC from Philadelphia.

A group of 100 Hollywood actors and producers wrote a letter of protest to NBC, saying that airing Trump's town hall was "enabling the President's bad behavior while undercutting the Presidential Debate Commission and doing a disservice to the American public."

The Biden campaign said on Thursday that two people involved in the campaign had tested positive for COVID-19, including one on the staff of Senator Kamala Harris, Biden's running mate.

Biden had also been on a plane with an aviation company employee who tested positive but was not in close contact, his campaign said in a statement. He tested negative for COVID-19 on Thursday as well as Wednesday, his campaign said.

While Harris was not in close contact with anyone who tested positive, the campaign said it was canceling Harris' travel until after Sunday as a precaution, although Biden's schedule will be unaffected.

"This shows how seriously we take COVID, how we have since March done everything in our power as a campaign to ensure the safety of our staff and volunteers and voters," Biden's campaign manager, Jen O'Malley Dillon, told reporters on a call.

Agencies via Xinhua