Americans on Sunday remembered the horror of September 11, 2001, and the nearly 3,000 people who died in the hijacked plane attacks as authorities worked to ensure the emotional 10th anniversary was peaceful.

God is our refuge and strength, President Barack Obama said, reading from Psalm 46, during the ceremony at Ground Zero, the centerpiece of which was the annual reading of the names of the dead by family members.

Thousands gathered on a clear Sunday morning to grieve where the twin towers of the World Trade Center once stood. With security high and no traffic, there was an eerie silence where a decade ago the 110-storey skyscrapers collapsed after being hit by hijacked airliners, sending a cloud over lower Manhattan.

The somber ceremony -- with bagpipes, youthful voices singing the national anthem and firefighters holding aloft a tattered American flag retrieved from Ground Zero -- drew tears. Family members wore T-shirts with the faces of the dead, carried photos, flowers and flags in an outpouring of emotion.

For the first time, relatives saw the just-finished memorial and touched the etched names of their dead loved ones. Some left flowers, others small teddy bears. Some used pencils to scratch the names on paper, others took a photograph.

Many wept as the names of the dead were read, by wives and husbands, fathers, mothers, sisters, brothers and children, some choked with emotion at their personal loss.

I haven't stopped missing my Dad. He was awesome, said Peter Negron, who was just a child when his father, Pete, was killed in one of the stricken towers. I wish my Dad had been there to teach me how to drive, ask a girl out on a date and see me graduate from high school and a hundred other things I can't even begin to name.

The al Qaeda attacks of 2001 are now such a part of American life that they have been included in the school curriculum. This was the first anniversary that included a U.S. president.

The memorial includes two plazas in the shape of the footprints of the Twin Towers with cascading 30 foot waterfalls. Around the perimeters of pools in the center of each plaza are the names of the victims of the September 11 attacks and an earlier 1993 attack at the trade center.

Ten years have passed since a perfect blue sky morning turned into the blackest of nights, New York Mayor Michael Bloomberg said during the ceremony.

Since then, we've lived in sunshine and in shadow, and although we can never unsee what happened here, we can also see that children who lost their parents have grown into young adults, grandchildren have been born and good works and public service have taken root to honor those we loved and lost.

Obama visited the North Memorial Pool, which sits in the footprint of the north tower. He walked around the pool hand-in-hand with first lady Michelle Obama, former President George W. Bush and his wife, Laura.

In a somber moment, Obama touched the names of the dead, engraved in stone, before he greeted some family members.

SECURITY ALERT

Police in New York and Washington were on high alert against a credible but unconfirmed threat of an al Qaeda plot to attack the United States again on the 10th anniversary.

Security was especially tight in Manhattan, where police set up vehicle checks on city streets as well as bridges and tunnels coming into the city. There was an unprecedented show of force in Manhattan from roadblocks on Times Square in midtown to the area around Ground Zero farther to the south.

It was our Pearl Harbor, said John McGillicuddy, 33, a teacher from Yonkers, New York, getting coffee and carrying two American flags on his way to the World Trade Center, referring to the Japanese attack that led America to join World War Two.

Every year, September is always rough, he said, as he prepared to grieve his uncle, Lieutenant Joseph Leavey, a New York firefighter who died in the south tower on September 11.

Pope Benedict prayed for September 11 victims and appealed to those with grievances to always reject violence as a solution to problems and resist the temptation to resort to hate.

In the September 11 attacks, 19 men from the Islamic militant group al Qaeda hijacked airliners and crashed them into the World Trade Center, the Pentagon outside Washington and a field in Shanksville, Pennsylvania. Just weeks after the attacks, U.S. forces invaded Afghanistan to topple that country's Taliban rulers who had harbored the al Qaeda leaders responsible for the September 11 attacks.

In Afghanistan, NATO-led forces said on Sunday that on the eve of the 10th anniversary a suicide bomber driving a truck of firewood attacked a NATO base in central Afghanistan in an operation for which the Taliban later claimed responsibility.

NATO said the Saturday afternoon attack killed four Afghan civilians and injured 77 NATO troops suffered injuries which were not life-threatening.

To mark the 10th anniversary, Obama was set to visit all three attack sites.

There should be no doubt: today, America is stronger and al Qaeda is on the path to defeat, Obama said in his weekly radio and Internet address on Saturday.

U.S. forces killed al Qaeda leader Osama bin Laden in Pakistan in May.

Sunday's Ground Zero ceremony had moments of silence marking when the planes hit the twin towers as well as when they collapsed. Other moments of silence will mark when a plane hit the Pentagon and another crashed in Shanksville after passengers fought back against the hijackers.

Sunday's ceremony was the biggest of a weekend of such events from coast to coast.

A decade later, after a faltering start, there are signs of rebuilding progress at the World Trade Center. The new One World Trade Center rises more than 80 stories above the ground as it inches to its planned 1,776 foot height -- symbolic of the year of America's independence from Britain.

The memorial plaza is ready and the neighborhood has enjoyed a revival, making it a trendy Manhattan place to live.