The unveiling of a new memorial honouring the passengers and crew who fought their hijackers aboard United Airlines Flight 93 on Sept. 11, 2001, was among several commemorative events that were held a day ahead of the anniversary of the 9/11 attacks on the U.S.

The first phase of the Flight 93 National Memorial, a concrete entry way, was dedicated on Saturday to the 40 people on board who died that day in a rural Pennsylvania field, near Shanksville, where the hijacked plane crashed.

It's believed the plane was being directed toward Washington, D.C.

Daniel Coughlin, the U.S. House chaplain at the time, called the sacrifices by those on Flight 93 "willing seed for freedom's harvest."

Vice-President Joe Biden and former presidents George W. Bush and Bill Clinton were among those attended the event. Canadian songstress Sarah McLachlan sang I Will Remember You.

Bush deemed it “the most courageous act in American history” by the crew and passengers and labelled it “the first counter-offensive on the war on terror.”

He warned against “the temptation of isolation” and invoked a “world of dignity and hope [in which] the United States of America lead the call for freedom.”

In his speech, Clinton said those on board Flight 93 gave the country "an incalculable gift" by preventing the hijacked plane from smashing into what he called "the centre of government."

The ex-president said he was appalled the memorial still needed another $10 million to be completed and said, off the cuff at the podium, that he and Republican House Speaker John Boehner had just pledged to create a bipartisan group to raise the remaining funds.

The final memorial will include a massive marble wall listing the names of those on Flight 93.

Obama at Arlington National Cemetery

In New York City, hundreds of people, including families and friends of victims from the Twin Towers, held hands in lower Manhattan as a bell clanged at 8:46 a.m. to signify the time the first hijacked plane crashed into the World Trade Center's North Tower.

Other Saturday events include a performance by the New York Philharmonic and a memorial by the Fire Department of New York for its 343 members who died on 9/11 and those who have died from illness after working at Ground Zero.

Obama, a state senator in Illinois at the time of the attacks, and his wife, Michelle, made a pilgrimage Saturday to Arlington National Cemetery just outside of D.C. to remember the dead of the Iraq and Afghanistan wars. members killed in the two wars.

The Obamas stopped to talk with members of a family who appeared to be visiting a grave. Afterwards, they strolled along one of the rows of tombstones.

On Sunday, the president is scheduled to visit New York City, Shanksville and the Pentagon. In the evening, he planned to speak at a memorial event at the Kennedy Center in Washington.

In his weekly address, the U.S. president focused on unity

“We will not succumb to division or suspicion," Obama said. "We are Americans, and we are stronger and safer when we stay true to the values, freedoms and diversity that make us unique among nations."

Obama thanked U.S. troops who have served in the post-Sept. 11 wars in Iraq and Afghanistan. He praised the military successes including the killing of al-Qaeda leader Osama bin Laden.

Similarly, former New York City Mayor Rudy Giuliani, who gave the weekly Republican address, said the 9/11 attackers failed to destroy the American spirit.

A mural dedicated to the memory of those who died in the 9/11 attacks on the World Trade Center is set up on the perimeter of the construction site in New York. Jim Young/Reuters

"The country was not broken, but rather, it was more united in the days after Sept. 11 than at any time in my lifetime," Giuliani said. "We displayed heroic spirit in many ways, but perhaps the most heroic was the unity of spirit that we shared as Americans”

Meanwhile, counterterrorism officials in the U.S. have been chasing a credible but unconfirmed tip that al-Qaeda has plans to set off a car bomb in New York City or Washington, with bridges or tunnels as potential targets

ABC News has reported that three individuals, one an American citizen, entered the U.S. in August aiming to carry out an attack.

With files from The Associated Press