Crowds of thousands joined police, firefighters, rescue workers and dignitaries Tuesday under wet grey skies at several solemn ceremonies in the United States and Canada to mark the sixth anniversary of the Sept. 11, 2001, attacks.

A crowd gathers in Zuccotti Park in lower Manhattan on Tuesday to commemorate the sixth anniversary of the attacks on the World Trade Center.
(Henny Ray Abrams/Associated Press)

Nearly 3,000 people died when hijacked planes crashed into the World Trade Center in New York City, the Pentagon in Washington, D.C., and a field in Pennsylvania.

Amid the backdrop of a rainy Manhattan skyline, the World Trade Center ceremony began with a traditional drum and bagpipe procession that escorted an honour guard of police and firefighters carrying the torn flag retrieved from the rubble of the twin towers.

Church bells sounded across the city at 8:46 a.m. ET as Mayor Michael Bloomberg led the crowd of relatives and mourners in a minute of silence to coincide with the time the first hijacked airplane hit the north tower.

"On that day we felt isolated, but not for long and not from each other," Bloomberg told the crowd gathered at a park near where the towers once stood.

"Six years have passed and our place is still by your side."

Firefighters and emergency workers who responded to the flood of 911 calls that came on Sept. 11, as well as construction workers who cleaned up the rubble, read aloud victims' names as those attending the service bowed their heads.

Many emergency workers died as they tried to rescue victims from the twin towers, while thousands more say they are still suffering from persistent respiratory problems because they inhaled toxic dust when the towers collapsed.

In Washington, U.S. President George W. Bush and his wife, Laura, marked the anniversary with a moment of silence from the South Lawn of the White House. Across the city, officials and relatives gathered at the rebuilt wall of the Pentagon, where American Airlines Flight 77 struck, killing 184 people inside the building and on the plane.

A smaller memorial honouring Flight 93's 40 passengers and crew began at 9:55 a.m., the time the airliner nosedived into an empty Pennsylvania field.

NYC ceremony away from Ground Zero

The list of victims was one name longer than in 2006 as the official death toll was increased this year, after New York City ruled that a woman's death by lung disease was caused by exposure to World Trade Center dust.

For the first time, the New York ceremony took place at Zuccotti Park near Ground Zero, but not at the site itself, as has been tradition.

The anniversary was moved because of the intense construction underway to build a memorial, skyscraper and transit hub at the site where the World Trade Center towers stood.

The CBC's Neil Herland, who covers the United Nations, lives three blocks from the World Trade Center site and said he can see the glow of the floodlights that constantly illuminate the area from his home.

"In many ways, it's an open wound in the heart of New York, an open wound that hasn't been filled in," Herland said Tuesday.

Some families protested the change in venue for the ceremony, so they are being allowed to lay flowers at the World Trade Center site.

"It's still like visiting a grave on the person's anniversary of their death," said Rosaleen Tallon, whose firefighter brother, Sean Tallon, died that day.

Giuliani promises no politics at ceremony

Other family members are upset that Rudy Giuliani, the city's former mayor, was allowed to speak at the ceremony because he is now a Republican presidential candidate.

Giuliani spoke at all previous anniversary events and had promised before this one that his appearance would not be political.

"It was a day with no answers, but with an unending line of people who came forward to help one another," he told those gathered on Tuesday.

Senator Hillary Rodham Clinton, the New York Democrat who is seeking her party's presidential nomination, also attended the New York ceremony.

The ceremony has drawn fewer mourners every year, and some question whether the event is excessive. Communities in New Jersey, where many of the victims lived, have scaled back their Sept. 11 events.

Families of Air India victims participate in memorial

Canadian Prime Minister Stephen Harper commemorated the Sept. 11 attacks while in Australia on Tuesday.

"Sept. 11, 2001, was truly a day that shook the world," he told the Australian Parliament in Canberra.

"Six years on, the horrific images from that morning still evoke anger, sorrow and, as intended, terror. The buildings may have been American, but the targets were every one of us."

He noted that 24 Canadians died in the attacks, while other Canadians, like Americans, have been affected by terrorist acts since Sept. 11, 2001.

"So both our countries have been bloodied by terror," Harper said. "And both of us are doing our part to confront and defeat it."

In Canada, those who lost loved ones in the attacks also held a memorial service in Toronto on Tuesday.

They were joined by the families of the victims of the 1985 Air India bombing and Public Safety Minister Stockwell Day, who read a tribute at the event.

Maureen Basnicki, whose husband Ken died in the collapse of the World Trade Center's north tower, has joined other victims' families in calling for an official memorial ceremony in Canada to mark the tragedy.

"It's like it happened yesterday," Basnicki told CBC News on Tuesday. "It certainly comes flooding back to me."

Canadian memorial considered

Day said the government is looking into the possibility of a national memorial.

Day said plaques commemorating the 9/11 attacks have already been erected on Parliament Hill and at Beechwood Cemetery in Ottawa, but that the Department of Canadian Heritage is reviewing a request for something more.

"It's very important to remember and it's important to remind people that Canada is not immune from threat and from risk, and seeing the family members of people who were killed … it reminds us of that," he said.

"That's the purpose, the benefit, not just of a service like this, but a memorial itself."

Day said the government is also working to enact legislation that would allow Canadian victims to sue foreign governments and groups that support terrorism.

With files from the Canadian Press