Democrat Hillary Clinton accused Republican Donald Trump of racism, sexism and tax avoidance during a heated presidential debate that could reshape the 2016 campaign for the White House.
Mr Trump, a real estate tycoon making his first run for public office, said Mrs Clinton's long years of service represented "bad experience" with few results and suggested her denial of a trade deal with Asian countries was insincere.
For Mr Trump, 70, the debate was a chance to appear disciplined. For Mrs Clinton, 68, it was an opportunity to reassure voters she could be trusted. It remained to be seen how voters would judge their performance.
In one of the more heated exchanges, the two candidates attacked each other for the controversy Mr Trump stoked for years over whether President Barack Obama was born in the United States.
The president, who was born in Hawaii, released a long form birth certificate in 2011 to put the issue to rest.
Only this month did Mr Trump say publicly that he believed Obama was US-born.
"He (Trump) has really started his political activity based on this racist lie that our first black president was not an American citizen. There was absolutely no evidence for it. Bu the persisted. He persisted year after year," Mrs Clinton said.
Mr Trump repeated his false accusation that Mrs Clinton's failed 2008 presidential campaign against Mr Obama had initiated the so-called "birther" issue.
"Nobody was pressing it, nobody was caring much about it ... I was the one that got him to produce the birth certificate and I think I did a good job," Mr Trump said.
African-American voters overwhelmingly support Mrs Clinton, but Mr Trump in recent weeks has said he believes his policy agenda would benefit them and said the policies of Mr Obama and Mrs Clinton had failed to help black Americans.
He said Mrs Clinton's arguments were disingenuous.
"When you try to act holier than thou, it really doesn't work," Mr Trump said.
Toward the end of the debate, Mr Trump said Mrs Clinton did not have the endurance to be president.
"She doesn't have the look, she doesn't have the stamina," he said.
Citing her own public record, Mrs Clinton retorted: "As soon as he travels to 112 countries and negotiates a peace deal, a ceasefire, a release of dissidents ... or even spends 11 hours testifying in front of a congressional committee, he can talk tome about stamina."
Each accused the other of distortions and falsehoods and urged viewers to check their campaign websites for the facts.
Mrs Clinton called the New York businessman's tax policies "Trumped-up trickle-down" economics and Trump accused the former secretary of state of being "all talk, no action."
"I have a feeling I'm going to be blamed for everything, "Clinton, the first woman to win the presidential nomination of a major U.S. political party, said during one tough exchange.
"Why not?" retorted Mr Trump, a former reality TV star.
Mrs Clinton knocked Trump for not releasing his income tax returns and said that decision raised questions about whether he was as rich and charitable as he has said. She noted that the few years of tax returns he had released showed that despite his wealth, he had paid no federal income tax.
"That makes me smart," Mr Trump said.
"I have a tremendous income," he said at one point, adding that it was about time that someone running the country knew something about money.
Mrs Clinton criticised Trump for failing to pay some of the business people with whom his company had contracted. She said she had met a lot of people who had been cheated by her opponent.
Mr Trump said such incidents of non-payment had taken place when the work was unsatisfactory.
Mr Trump attacked Mrs Clinton for her trade policies and said she would approve a controversial trade deal with Asian countries despite opposing it as a candidate.
"You were totally in favor of it, then you heard what I was saying, how bad it is, and you said, 'Well, I can't win that debate,' but you know that if you did win, you would approve that," he said.
Mrs Clinton rejected the criticism.
"Well Donald, I know you live in your own reality, but that is not the facts," she said.
Moderator Lester Holt struggled to rein in the candidates, with discussions about trade policy suddenly shifting to the fight against Islamic State as Mr Trump accused Mrs Clinton of giving away information to the enemy by revealing on her website how she planned to defeat the group.
Mrs Clinton said that unlike Mr Trump, she at least had a plan for fighting Islamist militants.
Opinion polls have shown the two candidates in a very tight race, with the latest Reuters/Ipsos polling showing Mrs Clinton ahead by four percentage points, with 41% of likely voters.
A second Reuters/Ipsos poll released yesterday showed half of America's likely voters would rely on the debates to help them make their choice.
The 90-minute debate could sway undecided and independent voters who have yet to make up their minds as well as voters from both parties who have tuned out the election until now.
The size of the television-viewing audience is expected to challenge the record of 80 million Americans who watched 1980's encounter between Democratic President Jimmy Carter and Republican Ronald Reagan.
Some commentators forecast Super Bowl-sized viewership of about 100 million people.
The debate, at Hofstra University on New York state's Long Island, was the first of three planned presidential debates and one vice presidential debate.