The World Trade Center flag is folded as friends and relatives of the victims of the Sept. 11, 2001, terrorist attacks gather for a commemoration ceremony on the eighth anniversary of the attacks. (Jason DeCrow/Associated Press)

U.S. President Barack Obama is calling on Americans to come together, serve their communities and strengthen the country in honour of the victims of the Sept. 11, 2001, attacks.

On Sept. 11, 2001, two hijacked planes crashed into the World Trade Center in New York City, one at the Pentagon in Arlington, Va., and a fourth in a field in Shanksville, Pa. Almost 3,000 people died in the attacks, including 24 Canadians.

"Let us renew the true spirit of that day — not the human capacity for evil but the human capacity for good," Obama said during a ceremony at the Pentagon on Friday. "Not the desire to destroy but the impulse to save and to serve and to build."

Obama issued a proclamation on Thursday honouring those who died and urging Americans to mark the anniversary with acts of community service. Sept. 11 was declared a national day of service earlier this year.

"We can summon once more that ordinary goodness of America, to serve our communities, to strengthen our country and to better our world," Obama said.

'Renew our resolve'

The president also called on Americans to "renew our resolve against those who perpetrated this barbaric act and who plot against us still."

U.S. President Barack Obama bows his head as he lays a wreath at the Pentagon memorial to the victims of the Sept. 11 attacks. (Charles Dharapak/Associated Press)"In defence of our nation we will never waiver in pursuit of al-Qaeda and its extremist allies," he said.

Four moments of silence, at 8:46 a.m., 9:03 a.m., 9:59 a.m. and 10:29 a.m. ET, marked the times when the first and second hijacked planes hit the World Trade Center buildings and when the south and north towers fell.

People carrying photos of victims wiped away tears and raindrops under grey, windy skies for the ceremony Friday morning in the plaza southwest of Ground Zero as bells tolled throughout New York City. Construction is occurring on the footprint site of the World Trade Center for a new tower, an underground museum and a memorial park.

The best way to "safeguard the memory of those who died" is by rekindling the spirit of volunteerism showed in the wake of the attacks, said New York City Mayor Michael Bloomberg.

"Eight years ago, countless people played a part in history by doing something to help another person — probably someone they didn't even know," said Bloomberg. "No one stopped to ask, 'If I can only do a little should I bother doing anything at all?' Each act was a length in a continuous chain that stopped us from falling into cynicism and despair."

Teresa Mathai, whose husband, Joseph Mathai, died at the World Trade Center, planned to grieve at a ceremony in Boston before volunteering for the rest of the day with Habitat for Humanity.

Remembrance overshadowed

The combination of mourning and national giving was troubling to some who feared the volunteerism would overshadow what they see as a sombre day to remember.

Relatives of victims stand before a ceremony marking the eighth anniversary of the Sept. 11 attacks on the World Trade Center on Friday. (Chris Hondros/Associated Press)"I greatly fear, at some point we'll transition to turning it into Earth Day where we go and plant trees and the remembrance part will become smaller and smaller and smaller," said Debra Burlingame, whose brother was the pilot of the American Airlines jet that crashed into the Pentagon.

Officials said fewer people than in previous years had gathered in the rainy weather to attend the annual ceremonies at the crash sites. Estimates put the attendance in the hundreds rather than the thousands.

The names of the victims are being read by family members and volunteers at the New York ceremony, which is being attended by U.S. Vice-President Joe Biden.

Obama and Defence Secretary Robert Gates are scheduled to meet with victims' family members later Friday.

International ceremonies

In Canada, a service was held in Appleton, N.L., near Gander where planes were diverted in 2001. Prime Minister Stephen Harper attended a remembrance ceremony at Beechwood cemetery in Ottawa.

In Afghanistan, meanwhile, about 1,000 American troops were participating in a memorial run that spanned 9.11 kilometres at Bagram Air Field.

In Britain, a wreath-laying ceremony was held in Grosvenor Square in London for the 67 British people killed.

With files from The Associated Press