The two candidates greeted each other with a handshake and a smile.
The two candidates greeted each other with a handshake and a smile to begin the highly anticipated debate.
As the debate opened, they put forward competing visions for the U.S. economy.
Clinton, a former secretary of state and the first woman to win the presidential nomination of a major U.S. political party, said Trump's tax policies were akin to "Trumped-up trickle-down" economics.
"The kind of plan that Donald has put forth would be trickle-down economics all over again. And in fact it would be the most extreme version, the biggest tax cuts for the top percents of the people in this country that we've ever had. I call it Trumped-up trickle-down, because that's exactly what it would be. That's not how we grow the economy," she said.
Trump, a real estate tycoon and former reality television star who has never held elective office, criticized Clinton for her trade policies.
"We have to stop our jobs from being stolen from us," he said.
Clinton, 68, wore a red pantsuit, and Trump, 70, wore a dark suit and a blue tie to the encounter that could shift the course of the tight 2016 race for the White House.
Opinion polls show the two candidates in a very tight race, with the latest Reuters/Ipsos polling showing Clinton ahead by 4 percentage points, with 41 percent of likely voters.
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.
A second Reuters/Ipsos poll released on Monday showed half of America's likely voters would rely on the debates to help them make their choice. More than half, 61 percent, were hoping for a civil debate and were not interested in the bitterness shown on the campaign trail.