Workers clearing rubble at Ground Zero stopped for the service, although fire hoses stayed in place as the ruins are still smouldering.
The smoking remains of the Twin Towers were surrounded by yellow, white and purple flowers and families held up photos of those who died.
Afterwards relatives were given an urn full of soil from the ruined site by Mayor Rudolph Giuliani, who said families found the service "comforting and supportive".
It's hard for many relatives as they haven't been able to have proper funerals. Many who died have not been found yet and it's the closest some of the friends and families have been to Ground Zero.