Across the country, Americans marked the 11th anniversary of the Sept. 11, 2001, terrorist attacks, tolling church bells, pausing in silence to reflect and mourning the loss of the nearly 3,000 people who died.

On the White House South Lawn, President Obama and first lady Michelle Obama bowed their heads at 8:46 a.m., the moment the first plane struck the North Tower of the World Trade Center. They later laid a wreath at the Pentagon, where the third plane struck. A flag was draped over the building to mark the day.

“Eleven times we have marked another September 11th come and gone. Eleven times, we have paused in remembrance, in reflection, in unity and in purpose,” Obama said to families and military brass who gathered at the Pentagon, where 184 were killed. “This is never an easy day.”

At Arlington National Cemetery, the Obamas visited the graves in Section 60, one of the sections where those killed in Afghanistan and Iraq are buried under white marble markers.

They placed a “challenge coin” on a collective memorial to the victims of an Oct. 29, 2009, helicopter crash in Afghanistan.

At a ceremony in Manhattan, where One World Trade Center is under construction, the families of victims read the names of loved ones killed in the attacks, and traders on the floor of the New York Stock Exchange stood silent.

In Shanksville, Pa., where United Airlines Flight 93 crashed in an open field after being hijacked by four terrorists, Vice President Biden spoke of the sacred, hallowed ground, weaving in his own experience of loss to comfort the families of the 40 passengers and crew members who died there.

“For no matter how many anniversaries you experience, for at least an instant, the terror of that moment returns; the lingering echo of that phone call; that sense of total disbelief that envelops you, where you feel like you’re being sucked into a black hole in the middle of your chest,” said Biden, whose first wife and baby daughter were killed in a car accident 40 years ago. “My hope for you all is that as every year passes, the depth of your pain recedes and you find comfort, as I have, genuine comfort in recalling his smile, her laugh, their touch.”

Thousands of miles away, in Kabul, troops prayed and reflected on the event that triggered America’s longest war.

In Reno, Nev., where he delivered a speech on foreign policy, Republican presidential hopeful Mitt Romney shared his own memories from Sept. 11, 2001, when he was in Washington as the head of the Winter Olympic Games, meeting with members of Congress about security preparations for the Salt Lake City Games.

He said he watched the second plane hit the World Trade Center on a small television on his desk. Later, he recalled, he left Washington and passed by the Pentagon just after he crossed the Potomac River.

“Cars had stopped where they were and people had gotten out, watching in horror,” Romney said. “I could smell burning fuel and concrete and steel. It was the smell of war, something I never imagined I would smell in America.”

Romney, who was criticized for his failure to mention U.S. troops or the war in Afghanistan in his nomination acceptance speech at the recent Republican National Convention in Tampa, said that he would help steer a century that began with terror, war and economic calamity onto the path of freedom, peace and prosperity.

“America must lead the free world, and the free world must lead the entire world,” Romney said. “In our dealings with other nations, we must demonstrate confidence in our cause, clarity in our purpose and resolve in the application of our military might.”

Although national security and foreign policy have taken a back seat to economic issues this election season, some conservatives were urging Romney to take a strong stance in the speech.

According to a Washington Post/ABC News poll, Obama has a clear advantage over Romney on foreign policy and national security. Four in 10 voters say the country is safer since Obama took office; only one in eight sees the country as less secure. And 51 percent of all voters polled said they trust Obama to handle terrorism; 40 percent said they trust Romney to do so.

On the steps of the U.S. Capitol, lawmakers put aside partisan bickering and gathered in prayer, remembering the day that changed the country and took so many lives.

“Every generation has endured hard sacrifice to preserve the blessings of freedom. If we ever falter, it will be because we forget what we learned in hardship,” said House Speaker John A. Boehner (R-Ohio), fighting back tears. “Today we listen, and vow to never forget. To celebrate the greater good that comes from serving one another and standing shoulder-to-shoulder. And to carry on, come what may, to meet the unmet challenges and complete the unfinished work.”

At Prince George’s Community College, faculty and students, of different countries and faiths, heeded the call to service.

As she placed index cards written by students on the wall of remembrance, Paulette McIntosh, a native of Jamaica, reflected.

“For me, seeing people of all different faiths and backgrounds coming together for a day of service is encouraging.”

Both presidential campaigns pulled their negative ads from television and radio out of respect for the day. Obama is expected to visit patients at Walter Reed National Military Medical Center later Tuesday.

“So as painful as this day is and always will be, it leaves us with a lesson that no single event can ever destroy who we are,” Obama said. “No act of terrorism can ever change what we stand for. Instead, we recommit ourselves to the values that we believe in, holding firmly, without wavering, to the hope that we confess.”

Philip Rucker in Reno, Peyton Craighill in Washington and Hamil R. Harris in Maryland contributed to this report.