New York (CNN) -- Al Qaeda's deadly attack on America 10 years ago solemnly reverberated Saturday on the eve of the anniversary, as Americans remembered the trauma and police stood guard from coast to coast amid ongoing fears of terror attacks.
President Barack Obama and the first lady visited Arlington National Cemetery outside Washington to pay their condolences to families mourning the loss of family members who served in the military.
They visited a section of the vast cemetery where people who served in Iraq and Afghanistan are buried. They embraced a young woman and walked hand in hand down a row of graves.
Obama proclaimed this weekend, Friday though Sunday, as National Days of Prayer and Remembrance and will be attending memorial services at the attack sites in New York, Washington and western Pennsylvania.
He also called on Americans to honor the victims of the terrorist attacks through activities such as prayer, memorial services, the ringing of bells and evening candlelight vigils.
"They wanted to terrorize us, but, as Americans, we refuse to live in fear," Obama said in his weekly address Saturday.
"Yes, we face a determined foe, and make no mistake -- they will keep trying to hit us again. But as we are showing again this weekend, we remain vigilant. We're doing everything in our power to protect our people. And no matter what comes our way, as a resilient nation, we will carry on."
Obama said the "United States must not relax its counterterrorism efforts in the weeks and months that follow."
A solemn memorial service at St. Patrick's Cathedral in New York included the recitation of the names of 343 firefighters who died at ground zero.
Patrick Mate Lyons, who was born Oct. 7, 2001, read an open letter to his father, Patrick, one of the casualties.
"I want you to know that Mommy is doing a great job of loving me and raising me in a happy home," Patrick said. "I play flag football in the same league as you, and in the same position as you, as quarterback. In baseball, I pitch, just like you did. I really like it when people compare me to you."
Fire Commissioner Salvatore Cassano said he recalled seeing a firefighter searching for his sister, who was killed. "It was then that the magnitude of the loss ... really began to sink into me. I realized the victims and their loved ones formed a far wider community than just the fire department."
Retired New York firefighter Rocco Chierichella recalled then-President George W. Bush's visit to ground zero on Sept. 14, 2001.
Chierichella yelled for Bush to speak louder to crews. "I can hear you, the rest of the world hears you," Bush said to cheers. "And the people who knocked these buildings down will hear all of us soon."
Chierichella told CNN that firefighters who arrived after the first World Trade Center tower was hit "didn't know this was an act of war." But they knew they were in a "war zone" after the second strike. "These firefighters didn't know what to expect (next)."
Meanwhile, about 5,000 people joined hands in an unbroken chain along Manhattan's West Side waterfront to remember the attacks.
"I felt it was important to recall and celebrate the generosity of spirit that united our community, city and nation," event organizer Julie Menin said.
"I am delighted by the overwhelming and immediate outpouring of support by individuals and companies who contributed so generously to the success of this living memorial. Each participant is making a very personal statement. Yet by their individual presence, they are a multitude becoming one."
At the Pentagon, Bush and his wife, Laura, laid a wreath of flowers by the 9/11 stone. Secretary of Defense Leon Panetta, former Defense Secretary Donald Rumsfeld and Joint Chiefs of Staff Chairman Adm. Mike Mullen also attended.
Bush also attended the dedication of the Flight 93 memorial in Shanksville, Pennsylvania. He was expected to join Obama in New York on Sunday.
Bush gave an emotional speech, retelling the story of how the passengers of Flight 93 made the decision to take on their captors, losing their lives so that others could be saved.
He called their action the first great counteroffensive of the war on terror.
"One of the lessons of 9/11 is that evil is real. And so is courage," Bush said. Theirs was "one of the most courageous acts in American history," he said.
Former President Bill Clinton echoed Bush's sentiments.
"They saved the Capitol from attack, they saved who knows how many lives," Clinton said at the site.
Also Saturday, New Jersey Gov. Chris Christie helped dedicate the Empty Sky Memorial at Liberty State Park in Jersey City. The memorial, which overlooks the Manhattan skyline and is in the shape of two rectangular towers, displays the name of 746 New Jersey residents who died in the attacks.