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The daughter of a police officer killed in the Sept. 11 attacks pauses during a ceremony Monday at the World Trade Center. ((Spencer Platt/Associated Press))

Fourmoments of silencewere held at Ground Zero in New York City on Monday to mark the fifth anniversary of the Sept. 11 attacks.

Thousands of people at the World Trade Center, where the majority of victims died,stoodquietly at 8:46 a.m. ET and 9:03 a.m. ET, when American Airlines Flight 11 and United Flight 175 hit the north and south towers respectively.

Those gathered at Ground Zerofell silent again at 9:59 a.m. ET and 10:29 a.m. ET, when the south and north towers fell. Ringing of bells preceded the moments of silence, while readings of the names of the victims followed.

New YorkMayor Michael Bloomberg noted at the ceremony howdifficult it must befor families of the dead to return to the site where their loved ones were killed on Sept. 11, 2001. The attacks by militants linked to al-Qaeda took the lives of nearly 3,000 people.

"Five years have come, and five years have gone, and still we stand together as one,"Bloomberg said.

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A tearful U.S. Staff Sgt. Alicia Watkins from Washington, D.C., salutes during a ceremony marking the fifth anniversary of the Sept. 11 attacks at a U.S. military base in Kabul, Afghanistan, on Monday. ((Musadeq Sadeq/Associated Press))

"We come back to this place to remember the heartbreaking anniversary — and each person who died here — those known and unknown to us, whose absence is always with us."

Spouses, partners and family members read the names of the dead.

Two Toronto women, Maureen Basnicki and Cindy Barkway, were expected to join those reading the names of loved ones who died in the attacks. Twenty-four Canadians died when the four planes crashed.

Basnicki's husband, Ken, was attending a conference on the 106th floor of the north tower when it was hit byone of the hijackedplanes. She told the Canadian Press in an interview that she was pleased to be asked along with her close friend, Barkway, to read names at the memorial.

"It's something that we're very, very proud to do and honoured, that's a better word —we're honoured," she said.

Dignitaries told the thousands gathered in New York City that the dead will never be forgotten.

"We've come back to remember the valour of those we've lost, those who innocently went to work that day and the brave souls who went in after them," said Rudolph Giuliani, who was mayor of New York at the time of the attack.

Laying wreaths

At the World Trade Center, many clutched photographs of their loved ones and many were crying. A few spouses of those who died told poignant stories of their partners and how they died.

In Washington, D.C.,men and women in uniform gathered at the Pentagon, where aplane was crashed into one wall of the building, while people marked theattacks in a field in Shanksville, Pa., where the fourth plane went down.

U.S. President GeorgeW. Bush and his wife, Laura,had breakfast with "first responders" in New York City early Monday and thentook part inthemoments of silenceto commemorate the anniversary.

In Shanksville,Bush and his wife stood and bowed their heads in front of a large wreath honouring the victims.

They later travelled to the Pentagon andlaid a wreath outside the reconstructed wall where 184 people were killed. The president and first lady later exchanged smiles and offered embraces to tearful family members gathered at the spot where American Flight 77 crashed into the defence headquarters five years ago.

In New York City, families will be allowed to descend a ramp to the lowest level of Ground Zero. Later at night, the Towers of Light will be illuminated, starting at dusk. The two beams of light have been illuminated in years past to mark the anniversary.

Memorial breakfast

In Canada, a number of events were being held to mark the day.

In downtown Toronto, a handful of family and friends of some of the 24 Canadians who died in the attacks had a memorial breakfast together.

Prime Minister Stephen Harper remembered the anniversary duringa televised speech in Ottawa, during which he also paid tribute to the more than 2,000 Canadian soldiers serving in Afghanistan.

With files from the Canadian Press and the Associated Press