In a presidential campaign debate filled with tension and insult, Hillary Clinton declared that Donald Trump's vulgar comments about women reveal "exactly who he is", while he accused her of attacking women involved in Bill Clinton's extramarital affairs and said she would "be in jail" if he were president.

Trump unleashed a barrage of attacks and acknowledged for the first time that he had paid no federal income taxes for many years.

The debate was the culmination of a stunning stretch in the race for the White House, which began with the release of a video in which Trump is heard bragging about how his fame allowed him to "do anything" to women.

Trump denied he had ever kissed or grabbed women without their consent. He said his words in 2005 were merely "locker room talk" and paled in comparison to what he called Bill Clinton's abuse of women.

Bill Clinton never faced any criminal charges in relation to the allegations, and a lawsuit over an alleged rape was dismissed. He did settle a lawsuit with one of the women who claimed harassment.

Clinton did not respond directly to Trump's accusations about her husband or her own role, but she condemned his comments about women in the tape and said she did not believe he "had the fitness" to be president.

Trump struggled at times to articulate detailed policy proposals, but he repeatedly cast Clinton as a career politician who had accomplished little during her years in Washington. He also said she should have been criminally charged for her controversial e-mail practices at the State Department.

Along with the new tape of Trump were revelations about Clinton's paid speeches to Wall Street firms. Emails released by WikiLeaks last week showed Clinton told a group that it's acceptable for a president to project differing positions in public and private.

The tension between Trump and Clinton was palpable from the start of their 90-minute debate, the second time they have faced off in the presidential campaign.

The candidates briefly put aside their animosity when asked by a voter if they respected anything about each other.

Clinton said she respected Trump's children, calling them "incredibly able and devoted".

Trump said Clinton "doesn't quit, she doesn't give up. I respect that".