NEW YORK — With the ringing of a bell, the thousands who gathered in Lower Manhattan fell silent on Sunday, and all that could be heard was the water cascading in the reflecting pools where the World Trade Center towers once stood.

It was 8:46 a.m., the moment when, 15 years earlier, a plane struck the north tower.

Family members of the victims bowed their heads, some crying and embracing, while others stood tall, looking up at the overcast skies.

About 8,000 people crowded the National September 11 Memorial plaza for what has become an annual ritual: the reading of the names of the 2,977 people killed in the ­terrorist attacks of Sept. 11, 2001, and of the six killed in the bombing of the World Trade Center on Feb. 26, 1993.

With the 15th anniversary falling on a Sunday, people turned out for the ceremony in much larger numbers than in previous years. But for the family members of the victims, it was another year of grief, and for some, the pain was as raw as ever.

“It feels like it’s been 15 seconds,’’ said Tom Acquaviva, whose son, Paul, died while working in the north tower. He was 29 years old, “too young,’’ Acquaviva said.

Classical string music played as mothers, fathers, siblings, children, and other relatives read the names, one by one. Some paused to say a few words to their loved ones; others offered updates on graduations, weddings, and other milestones during the past 15 years.

The bell tolled five more times: at 9:03, when a plane struck the south tower; at 9:37, when a plane hit the Pentagon, near Washington; at 9:59, when the south tower collapsed; at 10:03, when a hijacked plane crashed in Shanksville, Pa., after passengers staged a revolt; and at 10:28, when the other tower fell.

Both presidential candidates, Donald J. Trump and Hillary Clinton, attended the ceremony, though neither made public remarks and both agreed not to campaign for the day. Cheers welcomed Trump just before the ceremony started.

An hour and a half into the ceremony, Clinton felt overheated and left to rest in her daughter’s apartment.

Later in the day, her campaign said Clinton had been diagnosed with pneumonia on Friday.

At the Pentagon, President Obama delivered his last address commemorating the 184 victims there, and while his words were similar to remarks he has given before, they were a clear rebuke to the divisive rhetoric coursing through the presidential campaign.

Obama said that after the attacks, it was important for the country to not only defend its territory, “but also our ideals.’’

He said that groups like Al Qaeda and the Islamic State know that they cannot defeat a nation such as the United States, so their attacks are intended to cause fear that leads Americans to turn on one another and “change who we are or how we live.’’

“And that’s why it is so important today that we reaffirm our character as a nation — a people drawn from every corner of the world, every color, every religion, every background — bound by a creed as old as our founding, e pluribus unum,’’ Obama said.

In Shanksville, about 1,000 people attended the annual service at the Flight 93 National Memorial, honoring the 40 passengers and crew members who died there.

The site, about 60 miles southeast of Pittsburgh, is where the United Airlines flight crashed as passengers stormed the cockpit.

Addressing the victims’ families, Sally Jewell, the secretary of the interior, said: “You have known the terrible pain of loss. None of us would want to trade places with you, but we honor your sacrifice.’’