Hundreds of people assembled Sunday morning in Lower Manhattan to mark the 15th anniversary of the devastating terrorist attacks on the World Trade Center.

The ceremony to honor the thousands who died at the site of the twin towers began at 8:40 a.m. with the national anthem and a reading of the names of those who were killed in 2001 and in an earlier attack in 1993.

The crowd observed six moments of silence timed to memorialize significant moments on Sept. 11.

Many in attendance carried flags. Some held photos of loved ones lost in the attack. Some wore T-Shirts with images of officer workers, firefighters and police officers who died that day.

Presidential candidates Hillary Clinton and Donald Trump attended the event outside the National September 11 Memorial & Museum.

Clinton abruptly left the ceremony feeling “overheated,” but her staff later revealed that she had been diagnosed two days earlier with pneumonia.

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President Barak Obama delivered remarks outside the Pentagon, where more than 180 people died when terrorists crashed an airliner into the building during the coordinated attacks.

“Even as you’ve mourned, you’ve summoned the strength to carry on,” he said to the families of the victims. “You remind us that there’s nothing that Americans can’t overcome.”

At Sunday’s memorial service in New York, relatives of those who were killed, spoke briefly about their loss.

Jim Giaccone spoke of his older brother, Joseph, who worked for the Cantor Fitzgerald financial services firm on the 103rd floor of the north tower. The company lost 658 employees in the attack.

“Fifteen years are like 15 seconds, he said, according to CNN. “The hurt is still there. The hole is still there. You live on through your family. We miss every day. And everyone tell someone you love them today.”

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Mayor Bill de Blasio, who attended Sunday’s service, spoke a day earlier at a memorial service at St. Patrick’s Cathedral honoring the 343 firefighters who died on Sept. 11.

“We remember what the members of the FDNY did in the face of something that could not be imagined,” he said. “When we needed them the most, they showed us the best, the best in humanity.”