Donald Trump and Kamala Harris clashed over issues including abortion, the economy and illegal immigration.
In the first debate before the US election, Ms Harris claimed that people left Trump's rallies out of "boredom," an attack that seemed to rattle the former president.
Trump retaliated and accused Ms Harris of being a dangerous "Marxist" and "the worst vice president ever."
One of Trump's frequent references referred to Ms Harris seemingly allowing mass illegal immigration and claimed that the "dangerous" immigrant influx were eating people's pets.
"In Springfield, they're eating the dogs. The people that came in. They're eating the cats. They're eating the pets of the people that live there," he said.
Ms Harris also hit out at "Trump abortion bans" and said it was "unconscionable" that government should dictate what women do with their bodies.
During his president, Mr Trump installed three Supreme Court justices who helped turn over the constitutional right to abortion in 2022.
Trump defended the move during the debate, saying the issue had torn the country apart.
"Each individual state is voting. It's the vote of the people now. It's not tied up in the federal government. I did a great service in doing it," said Mr Trump.
Before the debate, an opinion poll by FiveThirtyEight out Ms Harris slightly ahead of Mr Trump at 47.2% to 44.4%.