The memorial service at Ground Zero began with a minute of silence on Saturday.
At 14.46 Swedish time, Manhattan fell silent.
That was when the first plane crashed into the northern twin tower 20 years ago.
Just over a quarter of an hour later, 15.03, it fell silent again.
Because that was when the second plane flew into the south tower 20 years ago.
At 15:37 and 15:59, it was quiet again in Manhattan, as well as at the Pentagon and Shanksville in Pennsylvania, where planes also crashed on September 11, 2001 and where memorial ceremonies were held on Saturday to honor the 2,977 victims.
Several presidents in place
When it was not quiet, their names were heard.
One by one, they were called out by relatives or defense employees who stepped out on stage at the assassination sites.
President Joe Biden, George W Bush, who was president at the time of the terrorist attacks, and former Presidents Barack Obama and Bill Clinton were on hand to honor the victims.
Artists Bruce Springsteen, Kelli O'Hara and Chris Jackson also performed during the day.
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20 years since the terrorist attacks on September 11 in the United States