Moments of silence, prayer and discord mingled with the roar of motorcycles, the echo of taps and a recording of Jimi Hendrix playing the national anthem on Wednesday as Washington marked the 12th anniversary of the Sept. 11, 2001, terrorist attacks.

On a sweltering summer day, perspiring Muslims and Christians argued on the Mall, bands of flag-bearing bikers cruised the city, and President Obama told relatives of Sept. 11 victims that “our hearts still ache for the futures snatched away, the lives that might have been.”

As the commemorations unfolded, the flags around the Washington Monument flew at half-staff, stirring in the southerly breeze.

A former Marine, Tobiath Steinmetz, clad in biblical garb, carried a full-size wooden cross on the Mall.

And Marco Price-Bey, a barber in Southeast Washington giving veterans free haircuts for the day, said, “It’s our social duty to serve and our moral obligation to look out for others.”

The events recalled the morning 12 years earlier when terrorists hijacked four jetliners, flying two into World Trade Center buildings in New York City and one into the Pentagon, and crashing one into a field near Shanksville, Pa.

About 2,800 people were killed in New York and Shanksville, and 184 died at the Pentagon.

“Today we remember not only those who died that September day,” Obama said during a ceremony at the National 9/11 Pentagon Memorial, which followed a moment of silence outside the White House.

“We pay solemn tribute to more than 6,700 patriots who have given their full measure since — military and civilians,” the president said. “We see their legacy in the friendships they forged, the attacks they prevented, the innocent lives they saved.”

“We pray for the memory of all those taken from us — nearly 3,000 innocent souls,” he told victims’ relatives. “Even more than memorials of stone and water, your lives are the greatest tribute to those that we lost.”

In New York, bells tolled as relatives read the names of victims at the memorial plaza on the site where the World Trade Center towers collapsed.

A similar scene played out in Shanksville, where the terrorists crashed a plane as the passengers stormed the hijackers.

On the Mall in Washington, before a ceremony marking the anniversary of the attacks, a small group that included Muslim Americans bickered with a small group of Christian activists over the role and merits of Islam.

A phalanx of park police on motorcycles were on hand to keep the groups separated.

“We’re just regular, God-fearing, Bible-believing, patriotic, red-blooded Americans,” said Ruben Israel, 52, of Los Angeles, who spoke for a group carrying banners that read, “You Need Jesus” and “Repent of Your Wicked Heart of Unbelief.”

“I think every Christian should be offended that they want to have a rally on 9/11,” he said, referring to the Muslims. “We’re here to promote Jesus Christ as the answer to that bloody religion.”

Isa Hodge, 39, of the American Muslim Political Action Committee, said his group had a permit from the National Park Service for the rally.

“Since the day of 9/11, that horrible event that happened . . . there’s been a fear campaign that’s been promoted over the last 12 years to make the word ‘Muslim’ synonymous with ‘terrorist,’ ” he said.

“The rhetoric has been so deeply ingrained in the American citizen that we’re the problem, Muslims are the problem,” he added. “We, as Muslims . . . are for the Constitution, the preservation of the Constitution, and the freedoms and the civil liberties.”

Hodge and Israel got into a face-to-face dialogue, which seemed to go beyond the sentiments of the day.

Discussions were refereed by Capt. P.J. Beck of the U.S. Park Police.

“They just want to be here to evangelize what they’re going to do,” he said of Israel’s group. “They want to do what they want to do,” he said of Hodge’s group.

“We’re here to make sure it all happens,” he said.

Scholar Cornel West appeared for the Muslim rally.

“We need to keep the focus on priceless and precious human beings, no matter who they are,” he said. “They could be in Syria. They could be in Somalia. They could be in Tel Aviv. They could be on the West Bank.”

“They could be U.S. drones doing it,” he said. “They could be gangsters who did it 12 years ago.”

In the background, loudspeakers blared Vivaldi and Hendrix, and a woman shouted, “Blessed are the peacemakers!”

Bystanders carried placards protesting drone strikes and an attack on Syria.

Retired Episcopal bishop George E. Packard of New York, a former Army platoon leader in Vietnam, gave a blessing at the rally: “May we give to America back a certain kind of peaceful presence on this day.”

Packard said afterward that he had been among the clergy summoned to the ground-zero field morgue in New York 12 years ago.

“I stood with the Muslim chaplain, and the Jewish chaplain, and the Catholic chaplain,” he said. “And we would say prayers over the small body parts we’d find, and there was a kind of brotherhood there.”

He lamented that Wednesday’s remembrances were so “strident.”

Meanwhile, motorcyclists who had been denied a permit to parade downtown en masse instead rode about in smaller, colorful groups to commemorate the day and protest their permit denial.

Roger W. Snuffer of Somerset, Pa., had ridden to Washington on a pristine motorcycle decked out in an American-style flag with the number 93 in the blue canton, in honor of United Airlines Flight 93, which crashed in Shanksville.

Snuffer said he was in the National Guard on Sept. 11, 2001.

“I actually had to go out to the site and recover pieces,” he said as he stood by his motorcycle on Constitution Avenue in a red, white and blue head bandanna. “So it means a lot to me about 9/11.”

“My opinion is, some of the people in the country have forgotten,” he said. “It’s something to remember and make sure it doesn’t happen again.”

Krissah Thompson and Mark Berman contributed to this report.