Kamala Harris went after her US presidential election rival Donald Trump's mental state and fitness for office on Tuesday after the 78-year-old Republican's televised town hall veered into a surreal, impromptu music session.

Three weeks ahead of the US election, Harris's campaign has begun to focus aggressively on Trump's health and mental stability, and was quick to weigh in, saying the ex-president appeared "lost, confused, and frozen on stage."

For about half an hour, Monday's event in Oaks near Philadelphia was standard fare ahead of the November 5 election, as Trump took friendly questions from supporters on the economy.

But a pause for two medical emergencies in the crowd turned into a bizarre 39 minutes of music and dancing as Trump abandoned the discussion of the election to put on his favourite hits, swaying awkwardly on stage.

"Who the hell wants to hear questions, right?" he said, bringing the Q&A section to an abrupt end and telling his people to crank up the volume.

Trump has made a brief, jerky dance his signature at the end of rallies for years, nearly always to his exit song – the Village People's 1978 disco anthem "YMCA."

On Monday, however, he stayed on stage for nine songs, ranging from opera to Guns N' Roses and Elvis, with the ex-president alternating his dance moves with standing in place and staring into the crowd.

"Hope he's okay," Harris opined dryly on X.

Undeterred, Trump was bopping to the Village People again on Tuesday at a rally in Atlanta, where he turned up 90 minutes late and didn't mention the previous night's events or the criticism.

Delivering his usual stump speech focusing on illegal immigration, Trump pushed his now-customary falsehoods and exaggerations about migrant crime in Colorado and Ohio and referred to the country as "occupied" by foreign criminals.

But if he was trying to draw a line under Monday's strangeness, he didn't succeed.

Trump called the two world wars "beautiful" and claimed to have invented the word "caravan." Explaining how he was going to make America great again, he lurched into an aside about his wealthy friends, beachwear and President Joe Biden.

"I have a lot of rich friends. They go here. They go there. They're boring as hell," he began, apropos of nothing in particular. "Nobody cares. They put on their bathing suit. They don't look a lot better than Biden."

Harris and Trump are locked in a dead heat, according to polls, and the election is set to be decided by seven swing states where the margins could come down to a few thousand votes each.

The Republican is the oldest person ever to be nominated for a presidential bid, after Biden dropped out of the race following a disastrous debate that sparked fears about his own age.

Trump has not released a recent comprehensive report on his state of health, prompting fierce criticism from Harris, 59, who has increasingly been homing in on Trump's age and physical and mental condition.

It was the topic of her closing argument as she sat down with popular radio host Charlamagne tha God in an effort to boost her messaging to black male voters – a part of the electorate where Trump has made gains.

After setting out her policies for improving the lives of African American men, she turned to Trump's rallies and repeated a claim that riled him during their September debate – that bored supporters were leaving his rallies early.

"This man is weak and he is unfit," Harris said.

Trump had begun the day with a less freewheeling tone at an Economic Club of Chicago event, where he said he was for slapping "obnoxious" tariffs on trading partners like Mexico so that companies move factories to the United States.

"To me, the most beautiful word in the dictionary is tariff," Trump said. (AFP)