President Donald Trump and his challenger Joe Biden have fiercely clashed in one of the most chaotic and bitter White House debates in years.
The US president was challenged over white supremacist support and refused to condemn a specific far-right group.
Opinion polls suggest Mr Biden has a steady single-digit lead over Mr Trump.
But with 35 days until election day, surveys from several important states show a closer contest.
Polls also suggest one in 10 Americans have yet to make up their mind how to vote. But analysts said Tuesday night’s debate – the first of three – probably would not make much difference.
What were the key moments?
Overall, the 90-minute debate in Cleveland, Ohio, was light on serious policy discussion. Both candidates talked over each other but Mr Trump cut in some 73 times, according to a count by CBS News.
The tenor became clear early on as the two candidates sparred over healthcare. Hectoring from Mr Trump saw Mr Biden call the president a “clown”.
As they moved on to the Supreme Court, the rancour continued, with Mr Biden refusing to answer when asked if he would try to expand the number of judges.
“Will you shut up, man?” the Democratic candidate snapped at Mr Trump, later adding: “Keep yapping, man.”
Mr Trump, the Republican candidate seeking a second term, responded: “The people understand, Joe. Forty-seven years [in politics], you’ve done nothing. They understand.”
In one of the most talked about exchanges of the night, the president was asked by the moderator, Fox News anchor Chris Wallace, if he was prepared to condemn white supremacists.
He initially said he would but when asked to denounce the far-right Proud Boys group by name, he sidestepped.
Mr Trump said: “Proud Boys, stand back and stand by, but I’ll tell you what, somebody’s got to do something about antifa and the left.”
The Proud Boys, an anti-immigrant, all-male group, took to social media to celebrate. “Standing down and standing by sir,” it posted on Telegram.
Antifa, short for “anti-fascist”, is a loose affiliation of far-left activists that often clash with the far right at street protests.
Earlier, Mr Biden said: “This is a president who has used everything as a dog whistle to try to generate racist hatred, racist division.”
In other moments:
- Mr Trump was asked if he would encourage his supporters to be peaceful if results of the election were unclear. “I’m encouraging my supporters to go into the polls and watch very carefully,” he responded
- When Mr Trump said Mr Biden would be at the behest of the left of the Democratic Party over health and environmental policy, Mr Biden responded: “I am the Democratic Party right now”
- Mr Trump defended his effort to swiftly fill a US Supreme Court seat to cement a 6-3 conservative majority on the bench, saying: “We won the election and we have the right to do it”
- The president brought up the past drug use of Mr Biden’s son, Hunter, who was discharged from the navy in 2014 for taking cocaine. Shouting over Mr Trump, Mr Biden said: “My son, like a lot of people, like a lot of people you know at home, had a drug problem. He’s overtaken it. He’s fixed it.”