By Andy Sullivan

Sept 30 (Reuters) - Interrupted repeatedly by President Donald Trump in their presidential debate on Tuesday, an exasperated Joe Biden resorted to insults and name-calling against an opponent who built his political career by coining belittling nicknames for his rivals.

"You're the worst president America has ever had," Biden, the Democratic presidential nominee, said during a back-and-forth on taxes.

The insults featured heavily during a chaotic encounter in which Trump often talked over Biden and moderator Chris Wallace. Biden's frustrations were frequently apparent.

"Will you shut up, man? This is so unpresidential," Biden said to Trump during a segment on the Supreme Court.

"The fact is this man doesn't know what he's talking about," Biden said during a discussion on healthcare.

Biden called the president a "clown" - twice. At one point, he apparently thought better of it.

"It's hard to get any word in with this clown, excuse me, this person," Biden during an exchange about taxes.

The former vice president called Trump a "racist" for banning racial-sensitivity training in his administration.

He called Trump "Putin's puppy," needling him for not confronting Russia about allegedly putting bounties on U.S. soldiers. Trump tried to object, but was reprimanded by Wallace for speaking out of turn.

On Twitter and at campaign rallies, Trump, 74, has called Biden "Sleepy Joe," an apparent dig at the 77-year-old Democrat's vitality. He did not use the nickname on Tuesday.

But in their first of three scheduled debates, Trump had plenty of disparaging things to say about his rival, whom he trails in national opinion polls ahead of the Nov. 3 election.

The president brought up Biden's son's struggles with drugs and mocked his opponent's academic performance at the University of Delaware, where according to the Washington Post, Biden graduated 506th in a class of 688.

"He was the lowest or almost the lowest in your class. Don't ever use the word smart with me," Trump said. "Because you know what, there's nothing smart about you, Joe."

Biden said he was proud of his son Hunter for overcoming addiction.

Trump also suggested that Biden could not drum up enough interest to hold large in-person rallies, when Biden criticized him for flouting social-distancing rules at campaign events.

"If you could get the crowds, you would have done the same thing," Trump said.

NBC "Nightly News" anchor Lester Holt said the debate "could have been a low point in political discourse."

"I'm a teacher. I would never in a million years let a classroom turn into this," Heather Cox Richardson, a history professor at Boston College, wrote on Twitter.

(Reporting by Andy Sullivan; Editing by Noeleen Walder and Peter Cooney)