BOSTON – On a day reserved for remembrance, President Bush and Sen. John Kerry stepped off the campaign trail Saturday to mark the three-year anniversary of the Sept. 11 attacks with warm words for the victims’ families.

At the ornate Boston Opera House, Kerry told the Massachusetts families who lost their relatives that day that “the American spirit endures.” In Washington, Bush said in a live radio address that the terrorist attacks were “a turning point for our nation.”

At the White House, the president assembled White House staffers and family members on the sunny South Lawn for a moment of silence before bringing family members of Sept. 11 victims and emergency response workers into the Oval Office for his radio address.

“We saw the goals of a determined enemy, to expand the scale of their murder, and force America to retreat from the world,” Bush said. “And our nation accepted a mission: We will defeat this enemy.”

Bush said the nation is safer than it was three years ago, but it is not safe yet.

“The United States is determined to stay on the offensive and to pursue the terrorists wherever they train or sleep or attempt to set down roots,” Bush said. “We will not relent until the terrorists who plot murder against our people are found and dealt with.”

In Boston, where the two planes that struck the New York’s World Trade Center originated, Kerry placed three white lilies tied with red, white and blue ribbons on the low granite memorial to the victims in the Boston Garden. As a small private plane towed a large American flag overhead, Kerry read the engraved inscription of the names of the Massachusetts victims.

Later, inside the Opera House for an emotional tribute, many of the victims’ family members praised Kerry for his support.

“For three years, he has been a compassionate presence at all of our family events,” said Linda Plazonja, referring to the Massachusetts 9-11 Fund’s activities.

Sonia Puopolo, whose mother was killed, said she would always be grateful for the kindness and love that Kerry and his wife, Teresa, showed her family.

“Thank you from the bottom of my heart,” Puopolo said. “I speak for my mother, Sonia, for asking and praying for a president, a president named John Kerry, to bring peace to a terribly troubled world.”

During a brief address to the families, Kerry said the American spirit “leads us to defy those who would harm us, and affirm that freedom will win.”

He said he understood that for the families the last three years had been hard. But he also cited the acts of kindness from doctors, rescuers and others who came together in the tragedy’s aftermath.

“So while Sept. 11th was the worst day we have ever seen, it brought out the best in all of us,” Kerry said. “We will always remember where we were on that day. And we must always remember that we will only defeat those who sought to destroy us by standing together as one America.”