Relatives of victims killed at the World Trade Center are observing a moment of silence to mark the seventh anniversary of the September 11 terrorist attacks.
The silence marks the exact time, 8:46, when the first hijacked plane struck the trade center. Other moments of silence were planned for the times when the second plane hit and when the towers fell. In Washington, President George W Bush led a White House gathering in observing the moment.
Standing next to the president under threatening skies for the brief South Lawn ceremony were his wife, Laura, and Vice President Dick Cheney and his wife, Lynne. Other ceremonies are being held throughout the day, including one at the Pentagon, where a new memorial will be dedicated. Services will also be held in Shanksville, Pennsylvania, where one of the hijacked planes crashed.
Later Thursday, presidential rivals Barack Obama and John McCain are due at ground zero to pay silent respects.