The presidential candidates go head-to-head in their first election debate in front of an expected television audience of 100 million.
The 90-minute debate comes six weeks before Election Day, with polls showing an extremely tight race.
Donald Trump has said Hillary Clinton's economic plan is "all talk, no action, sounds good, never going to work".
Donald Trump has said he will release his tax returns if Hillary Clinton discloses her "33,000 emails". He said: "In a way I should be complaining. I don't mind it. It's almost become a way of life.
"I will release my tax returns against my lawyer's wishes. When she releases 33,000 emails that have been deleted."
In reply, Mrs Clinton said: "I made a mistake releasing private emails. It was a mistake and I take responsibility for that.
"I don't think Trump will ever release his tax returns. He must be hiding something".
Hillary Clinton has been mocked by Donald Trump for letting her take the blame for "everything".
She said: "I have a feeling that by the end of the evening I'm going to be blamed for everything that's ever happened."
Donald Trump hit back and said: "Why not?"
Donald Trump has said America must do a "better job" at helping companies expand.
"We have to do a much better job at keeping our jobs. We have to do a much better job at giving companies incentives to build new companies expand.
"Hillary I'll ask you this - you've been doing this for 30 years, why are you just starting to think of solutions now?
"I will bring back jobs. You can't bring back jobs", he said.
In reply, Mrs Clinton said: "I have thought about this. When I was in the Senate, I had a number of trade deals come before me and I gave them the same test. New jobs with rising incomes."
Donald Trump has blamed China for currency devaluation and US job losses.
He said: "You look at what China's doing to our country in terms of making our product. They're devaluing they're currency and there's nobody in our government to fight them."
Hillary Clinton has said she wants to make the US stronger by making the economy stronger.
She said: "We have to make the economy stronger. That starts with raising the national minimum wage and also guarantee - finally - equal pay for woman's work."
"The more we can do for the middle class, the more we can do for you. Trump stands for 'Trumped up' trickle-down economics", she added.
Donald Trump and Hillary Clinton have greeted each other on stage.
The three topics for the US election debate have been decided as achieving prosperity, America's direction and securing America.
The host of the US presidential debate, Lester Holt, arrives on stage at Hofstra University in Hempstead, New York.
Donald Trump's wife Melania Trump and Hillary Clinton's husband Bill Clinton shake hands as the presidential election debate nears.
