Many struggled to hold back tears at the ceremony held at Ground Zero, the site of the Twin Towers destroyed in the attacks by al-Qaeda militants.
"Twenty years feels like an eternity, but it still feels like yesterday," cried Lisa Reina who lost her husband.
A minute's silence was held at the exact time each hijacked plane crashed.
George W Bush, who was the US president at the time, gave a speech in Pennsylvania, where one of the planes crashed into a field after passengers overpowered the hijackers.
