On Thursday the day the US celebrated the seventh anniversary of the 2001 terrorist attacks was also the day when the US presidential election campaign was put on ice out of respect for the victims of the crashes.

The nominees participated in some of the formal commemoration activities at the sites where 19 hijackers crashed passenger planes into the symbols of US financial and military might on that September day.

John McCain, 72, the Republican presidential nominee, spoke at the crash site of Flight 93 in Shanksville, Pennsylvania, where passengers attempted to regain control of the plane to prevent it from being used as a human-guided missile to strike Washington, possibly even the US Capitol Building, which some believe was its intended target.

McCain saluted those on board for their courage, saying they may have saved hundreds of people on Capitol Hill, possibly even the Arizona senator himself. Forty passengers died in the crash.

They "deprived hateful enemies of their triumph," McCain said.

McCain and Democratic nominee Barack Obama, 47, agreed earlier this week to suspend campaigning and advertising on Thursday.

The two senators made a joint appearance at Ground Zero in New York, where nearly 3,000 people were killed after two hijacked planes crashed into the World Trade Center.

The candidates conversed quietly as they walked together down the ramp into what remains a hole in the ground. Obama and McCain were followed by McCain's wife, Cindy, and New York Mayor Michael Bloomberg.

The candidates each laid a flower in the memorial reflective pool, where hundreds of blooms had been placed earlier Thursday by the victims' families.

The candidates also greeted onlookers and local police officers, thanking them for their service.

McCain and Obama later joined an evening event at Columbia University labelled as a "presidential forum" on service.

Both tried to maintain a positive tone in separate discussions, linked by a handshake, greeting and embrace between the two candidates as Obama came on stage after McCain was finished.

Both praised citizens who volunteer for military service, as well as civilians who spend their free time engaged in social causes.

McCain and Obama both said that they would have pushed for more service and sacrifice by the US people during the period after the September 11 attacks.

Obama met Thursday with former president Bill Clinton at his charitable foundation's offices in the Harlem district of New York City. The two talked about the campaign, the economy and "how the world has changed since September 11, 2001," according to a joint statement.

Clinton was slow to support Obama after his wife Hillary's bid for the Democratic presidential nomination ended, but gave a rousing endorsement speech at the party nominating convention in late August.

Clinton plans to campaign for Obama later this month, the statement said.

Back in Washington, a memorial was dedicated at the Pentagon Thursday by US President George W Bush - the first memorial to be finished. The fourth plane crashed into the Pentagon in Washington, killing 184 people in the plane and on the ground.

Memorials in Shanksville and at New York's Ground Zero are to be finished by the 10th anniversary in 2011.