On a September morning eerily similar to the same day 10 years ago, Memorial Church rang its bells on Sunday at 8:46 a.m., 9:03 a.m., 9:37 a.m., and 10:06 a.m. to commemorate the times when two hijacked plans slammed into the World Trade Center, another smashed into the Pentagon, and a fourth crashed in a Pennsylvania field. With the tolling of the second bell, a girl crossing Tercentenary Theatre paused to make the sign of the cross.

And across the country, Americans gathered to remember the tenth anniversary of the terrorist attacks of September 11, 2001—a day that claimed more than 3,000 lives.

In New York City, more than 10,000 victims’ relatives gathered at what is now called ground zero, where political leaders descended to address the day’s significance.

“Friends and neighbors; sisters and brothers; mothers and fathers; sons and daughters—they were taken from us with heartbreaking swiftness and cruelty,” President Barack Obama said. “On Sept. 12, 2001, we awoke to a world in which evil was closer at hand, and uncertainty clouded our future.”

Obama spoke of patriotism and resilience, citing the heroism of the passengers of United Airlines 93, who took control of one of the hijacked plane, crashing it into a field near Shanksville, Pa.

“Shanksville is the scene of friendships forged between residents of that town, and families who lost loved ones there,” Obama said. “Where the World Trade Center once stood, the sun glistens off a new tower that reaches toward the sky.”

Former New York Mayor Rudolph Giuliani dismissed the terrorists’ attempt to “destroy America’s spirit,” in Saturday’s weekly Republican address.

“As we consider the rescue and recovery effort we witnessed at the time of and in the aftermath of the attacks, it’s clear that the terrorists failed,” said Giuliani, who became known as “America’s Mayor” in the aftermath of the attacks.

During the ceremony at ground zero, family members read off the names of the victims, sometimes speaking briefly about their deceased loved one.

In one address, a young boy, Nicholas Gorki, recalled his father, “who I never met because I was in my mother’s belly. I love you, Father. You gave me the gift of life, and I wish you could be here to enjoy it with me.”

Another, the wife of a police officer who died in a rescue attempt, said how every year, the memorial became more significant.

“My kids are 25, 21, 18. They understand now,” Barbara Gorman said at a service for the Port Authority dead. “It’s not so much a tragedy anymore as history, the history of our country.”

The speeches were delivered at the new memorial of the attacks­—two 30-foot deep pools that stand in the twin towers’ footprints.

Throughout the day, family members of those who died in the attacks traced the names of their relatives, which are etched into the bronze panels on the pools’ borders.

At the Pentagon, Vice President Joseph Biden praised the “9/11 generation” as one of history’s greatest.

“Never before in our history has America asked so much over such a sustained period of an all-volunteer force,” he said. “So I can say without fear of contradiction or being accused of exaggeration, the 9/11 generation ranks among the greatest our nation has ever produced, and it was born—it was born—it was born right here on 9/11.”

—The Associated Press contributed to this article.

—Staff writer Gautam S. Kumar can be reached at gkumar@college.harvard.edu.