Presidential politics took a back seat to grief and remembrance Tuesday, as candidates from both parties observed the sixth anniversary of the Sept. 11, 2001 terrorist attacks.

Under gloomy gray skies, two presidential hopefuls — Republican Rudy Giuliani and Democrat Hillary Rodham Clinton — attended a somber ceremony near ground zero. It was led by New York Mayor Michael Bloomberg, another potential White House contender.

Giuliani, the former New York mayor whose leadership in the wake of the attacks catapulted him to the top of the GOP field, delivered brief remarks just after a bell was rung to commemorate the collapse of the World Trade Center's South Tower.

"In the midst of our great grief and turmoil, we also witnessed uncompromising strength and resilience as a people," Giuliani said.

The former mayor also quoted famed author and Holocaust survivor Elie Wiesel.

"I have learned two lessons in my life: First, there are no sufficient literary, psychological or historical answers to human tragedy, only moral ones," Giuliani said. "Second, just as despair can come to one another only from other human beings, hope, too, can be given to one only by other human beings."

Giuliani and his wife, Judith, huddled together under an umbrella before he stepped to the podium to speak. He shared the stage with firefighters and other emergency responders who had been invited to read the names of the dead.

While Giuliani has participated in the city's 9/11 commemoration ceremony each year, many victims' families said Giuliani should not appear this time because he is running for president.

Clinton, a New York senator, did not speak at the ceremony and left shortly after it began to return to Washington for Senate business.

Clinton's lead rival, Sen. Barack Obama, D-Ill., released a statement calling for the country to "recapture that sense of common purpose" shared in the days following the terrorist attacks. He also indirectly criticized the Bush administration, noting that 9/11 mastermind Usama bin Laden was still at large.

"Six years later, the threat to America has only grown," Obama said.

Democrat Bill Richardson also released a statement calling on President Bush and the Congress to strengthen intelligence gathering and pursue a foreign policy that will eradicate terrorism.

"National security cannot and should not be about partisan turf wars," Richardson said.

Democrat Chris Dodd also invoked politics in a statement commemorating the occasion.

To prevent future attacks, the Connecticut senator said the U.S. must end the Iraq war and "reinforce our efforts to secure Afghanistan and hunt down the real terrorists responsible for the 9/11 attacks."

Republican Mitt Romney, another presidential contender, released a statement describing the attacks as the day "radical Islamists brought terror to our shores" and paying tribute to U.S. troops sent to Afghanistan and Iraq in the aftermath.