The 77-year-old Biden also responded forcefully to a suggestion by the questioner that he was 'too old' to be running for president.
Washington: Democratic presidential candidate Joe Biden snapped at a questioner during a campaign event in Iowa on Thursday after the man asked about his son's business dealings in Ukraine.
"You're a damn liar, man," the former vice president said in a rare flash of anger at the town hall in New Hampton, Iowa.
After first saying that President Donald Trump was "messing around in Ukraine," the man told Biden "you, on the other hand, sent your son over there, to get him a job and work for a gas company.”
"He had no experience," the man said. "You're selling access to the president."
"That's not true and no one has ever said that," Biden responded after calling the man a liar.
Biden's son Hunter served on the board of Ukrainian gas company Burisma from 2014 to 2019 and was reportedly paid USD 50,000 a month.
Hunter Biden has not been accused of any wrongdoing but questions have been raised about the propriety of him taking the position while his father was vice president and dealing with Ukraine issues.
Trump is facing impeachment in the Democratic-controlled House of Representatives after asking Ukrainian President Volodymyr Zelensky to investigate the Bidens.
The 77-year-old Biden also responded forcefully to a suggestion by the questioner that he was "too old" to be running for president.
"I'm not sedentary," Biden said. "The reason I'm running is because I've been around a long time and I know more than most people know and I can get things done.
"You want to check my shape on it, let's do pushups together, man," he said. "Let's run. Let's do whatever you want to do. Let's take an IQ test."
At another point, he appeared to say "Look, fat, look, here's the deal" — but this was strongly disputed by Biden aide Symone Sanders, who tweeted that he had said: "Look, facts."
Biden is the current frontrunner in the race for the 2020 Democratic presidential nomination, according to an average of national polls by RealClearPolitics.com.
Massachusetts Senator Elizabeth Warren, Vermont Senator Bernie Sanders and South Bend, Indiana, mayor Pete Buttigieg are the only other candidates in the crowded field with double-digit support.
The first vote of the Democratic primary season is to be held in Iowa in February.