The debate was heated from the start, with Trump frequently trying to interrupt Clinton and speaking over her answers. Clinton was more measured and restrained, but also needled the sometimes-thin-skinned Trump over his business record and wealth.
"There's something he's hiding," she declared, scoffing at his repeated contention that he won't release his tax returns because he is being audited.
Trump aggressively tried to turn the transparency questions around on Clinton, who has struggled to overcome voters' concerns about her honestly and trustworthiness. He said he would release his tax information when she produces more than 30,000 emails that were deleted from the personal internet server she used as secretary of state.
HEMPSTEAD, N.Y. (AP) -- In a combative opening debate, Hillary Clinton emphatically denounced Donald Trump Monday night for keeping his personal tax returns and business dealings secret from voters and peddling a "racist lie" about President Barack Obama. Businessman Trump repeatedly cast Clinton as a "typical politician" as he sought to capitalize on Americans' frustration with Washington.
Locked in an exceedingly close White House race, the presidential rivals tangled for 90-minutes over their vastly different visions for the nation's future. Clinton called for lowering taxes for the middle class, while Trump focused more on renegotiating trade deals that he said have caused companies to move jobs out of the U.S. The Republican backed the controversial "stop-and-frisk policing" tactic as a way to bring down crime, while the Democrat said the policy was unconstitutional and ineffective.
Tax experts have said there is no reason the businessman cannot make his records public during an audit.
Clinton was contrite in addressing her controversial email use, saying simply that it was a "mistake". She notably did not fall back on many of the excuses she has often used for failing to use a government email during her four years as secretary of state.
"If I had to do it over again, I would obviously do it differently," she said.
The candidates sparred over trade, taxes and how to bring good-paying jobs back to the United States.
Clinton said her Republican rival was promoting a "Trumped-up" version of trickle-down economics -- a philosophy focused on tax cuts for the wealthy. She called for increasing the federal minimum wage, spending more on infrastructure projects and guaranteeing equal pay for women.
Trump panned policies that he said have led to American jobs being moved overseas, in part because of international trade agreements that Clinton has supported. He pushed Clinton aggressively on her past support for the Trans-Pacific Partnership trade pact while she was serving in the Obama administration. She's since said she opposes the sweeping deal in its final form.
"You called it the gold standard of trade deals," Trump said. "If you did win, you would approve that."
Disputing his version of events, Clinton said, "I know you live in your reality."
"He has really started his political activity on this racist lie," Clinton charged.
Clinton aides spent the days leading up to the debate appealing for the media and voters to hold Trump to a higher standard than they believe he has faced for much of the campaign. Their concern was that if the sometimes-bombastic Trump managed to keep his cool onstage, he'd be rewarded -- even if he failed to flesh out policy specifics or didn't tell the truth about his record and past statements.
Trump's campaign has said the Clinton camp's concerns reflected worries about the her debating skills.
The centerpiece of Trump's campaign has been a push for restrictive immigration measures, including a physical wall along the U.S.-Mexico border and an early proposal to temporarily bar foreign Muslims from coming to the U.S. But he's been less detailed about other ideas, including his plan for stamping out the Islamic State group in the Middle East.
Clinton, a former senator and secretary of state, is banking on voters seeing her as a steady hand who can build on the record of President Obama, whose popularity is rising as he winds down his second term in office. She's called for expanding Obama's executive orders if Congress won't pass legislation to overhaul the nation's immigration system and for broader gun control measures. Overseas, she's called for a no-fly zone in Syria but has vowed to keep the military out of a large-scale ground war to defeat the Islamic State group.
For Clinton, victory in November largely hinges on rallying the same young and diverse coalition that elected Obama but has yet to fully embrace her.
Trump has tapped into deep anxieties among some Americans, particularly white, working-class voters who feel left behind in a changing economy and diversifying nation. While the real estate mogul lacks the experience Americans have traditionally sought in a commander in chief, he's banking on frustration with career politicians and disdain for Clinton to push him over the top on Election Day.
During the primary
Trump was often a commanding presence in the Republican primary debates, launching biting personal attacks on his rivals. But at other times, he appeared to fade into the background, especially during more technical policy discussions -- something he'll be unable to do with just two candidates on stage.
Clinton has debated more than 30 times at the presidential level, including several one-on-one contests against Obama in 2008 and Vermont Sen. Bernie Sanders in 2016. But Monday's contest will be her first presidential debate against a candidate from the opposing party.