Donald Trump sought to undermine Tuesday the credibility of voting in the biggest city of must-win US state Pennsylvania, an unsupported claim quickly and firmly denied by top Philadelphia officials.

Amid reports of exceptionally high voter turnout in a soundly Democratic area, Trump said there was "a lot of talk about massive cheating" in the city.

"Law enforcement coming!!" Trump wrote on his Truth Social platform.

A city official promptly denied the claim, calling it "yet another example of disinformation," while Philadelphia police and the district attorney's office also rejected the unsubstantiated allegation.

"There is no factual basis whatsoever within law enforcement to support this wild allegation," said District Attorney Larry Krasner.

"If Donald J. Trump has any facts to support his wild allegations, we want them now," he added.

Trump provided no evidence to support his claim made as Americans voted in a tense election that polls have suggested is effectively tied between Trump and Kamala Harris.

"There is absolutely no truth to this allegation," said City Commissioner Seth Bluestein, a Republican.

"Voting in Philadelphia has been safe and secure."

Trump rejected his loss in the 2020 election to Joe Biden, a denial that culminated with supporters of the ex-reality TV star violently attacking the US Capitol in a bid to block certification of the vote.

Trump is expected to reject the result if he loses again this year, raising the specter of chaos and violence in a tense and deeply polarized United States.

The Republican has already started raising questions about the integrity of the vote in Pennsylvania, a battleground state that is a key prize in the battle for the White House.

Pennsylvania's Democratic Governor Josh Shapiro, in a CNN interview last week, rejected Trump's allegations, noting that as the state's attorney general Shapiro defeated 43 challenges to the 2020 vote count from the ex-president and his allies.

When asked Tuesday whether he would accept defeat in the 2024 election, Trump offered a qualified response.

"If I lose an election, if it's a fair election, I would be the first one to acknowledge it. So far I think it's been fair," he said.