NEW YORK (AP) — For almost a decade, the anniversary of the Sept. 11 terrorist attacks was marked by somber reflection and a call to unity, devoid of politics. Not this time.

This year’s commemoration of the attacks that killed nearly 3,000 people in New York, Washington and Shanksville, Pa., promises to be the most political and contentious ever because of a proposed Islamic center and mosque near ground zero and a Florida pastor’s plan to burn the Quran — and the debate those issues have engendered over religious freedom.

As in other years, official ceremonies are planned at the three locations the terrorists struck. President Barack Obama will attend a commemoration at the Pentagon, while Vice President Joe Biden will attend the ceremony at ground zero. First lady Michelle Obama and former first lady Laura Bush will travel to Shanksville to observe the ninth anniversary there.

Obama told a White House news conference that Sept. 11 would be “an excellent time” for the country to reflect on the fact that there are millions of Muslims who are American citizens, that they also are fighting in U.S. uniforms in Afghanistan, and “we don’t differentiate between ‘them’ and ‘us.’ It’s just ‘us.'”

He said a plan by Terry Jones, the pastor of a small, independent church in Gainesville, Fla., to mark 9/11 by burning copies of the Quran must be taken seriously because it could cause “profound damage” to U.S. troops and interests around the world.

“You don’t play games with that,” Obama said, adding that as commander in chief he had an obligation to respond.

Jones has been under pressure from the White House and Defense Secretary Robert Gates to cancel the event. He said he called it off and wouldn’t follow through with burning the Muslim holy book if he was able to meet on Saturday with Imam Feisal Abdul Rauf, who is leading an effort to build the Islamic center and mosque near ground zero. Rauf said in a statement Friday he has no plans at this time to meet with Jones, although he is open to seeing anyone “seriously committed to pursuing peace.”

Obama said he hopes Jones “prays on it and refrains from doing it,” referring to the pastor as “the individual down in Florida.”

In Afghanistan, 11 people were injured Friday in scattered protests of Jones’ plan. Only a few thousand people attended those rallies and no large-scale demonstrations were reported elsewhere. In Indonesia, the world’s most populous Muslim country, cleric Rusli Hasbi told 1,000 worshippers at Friday prayers that whether or not he burns the Quran, Jones had already “hurt the heart of the Muslim world.”

Jones’ daughter, Emma, said in an interview with the German news website Spiegel Online that she begged him in an e-mail, “Papa, don’t do it,” but he didn’t answer. She said she hasn’t had contact him with since 2008, when he was ousted by members of a church he had founded in Cologne, Germany.

Biden will attend the largest of the three 9/11 commemorations — the New York ceremony at a park near ground zero, where 2,752 people were killed when Muslim extremists flew planes into the twin towers of the World Trade Center in 2001. The ceremony there will pause four times: twice to mark the times each plane hit the towers, and twice to observe the times the towers fell. Houses of worship in the city have been asked to toll their bells at 8:46 a.m., when the first plane struck the north tower.

But this time, along with the formal ceremonies, activists for and against the proposed Islamic center are planning their own events to capture the emotion of the day for political purposes.

Also Saturday, former Republican vice presidential candidate Sarah Palin was expected to observe the anniversary in Alaska with Fox News TV host Glenn Beck. The two conservative celebrities hosted a tea party rally last month at the Lincoln Memorial.

Nowhere do emotions run higher than in New York, where the proposed Islamic center just two blocks north of ground zero has inflamed passions before the commemoration.

The mosque site was locked and vacant Friday. Police officers guarding the block said the building would be closed through Saturday and worshippers who normally attend services there had been directed to a different prayer room about 10 blocks away.

Activists are organizing a pair of rallies — one against the planned Islamic center, one supporting it — to follow New York’s official ceremony at a park southeast of the trade center site.

The anti-mosque rally has bitterly divided family members of those who died in the attacks, with some planning to attend the rally and speak, while others denounce it as unnecessary and wrong.

Sally Regenhard, who lost her firefighter son, Christian Regenhard, in the attacks, said she would attend the city ceremony in the morning where the names of the dead are read aloud, as she has done each year since the attacks. Then, she planned to head over to the anti-mosque rally.

“The purpose is to speak out and express our feelings that this mosque, the location of it, is a grievous offense to the sensitivity of 9/11 families,” Regenhard said. “There’s nothing political about people who want to speak out against something they think is so wrong, so hurtful and so devastating.”

But Donna Marsh O’Connor, whose pregnant daughter, Vanessa, was killed in the attacks, supports the mosque. She said she strongly opposes the planned rally and the political motivations behind it.

“It’s more of the same hate mongering and fear mongering that’s been going on for years,” O’Connor said. “People have a right to free speech. But if they’re talking about sensitivities to 9/11 families, why are they rallying and doing events on a day we should spend thinking about those we lost?

The rally is being hosted by Pamela Geller, a conservative blogger who has actively opposed the planned Islamic center since its inception.

John Bolton, who served as U.S. ambassador to the United Nations under President George W. Bush, was expected to send a videotaped message of support to the rally, as was conservative blogger Andrew Breitbart. Anti-Islam Dutch politician Geert Wilders, who advocates banning the Quran and taxing Muslim women who wear head scarves, plans to address the crowd in person, as do a handful of Republican congressional candidates who have made opposition to the mosque a centerpiece of their campaigns.

In an interview with The Associated Press, Geller said the rally would be “respectful” and was not intended to provoke violence or other inappropriate behavior on what has typically been a somber, mournful anniversary.

“It’s a rally of remembrance for tens of thousands who lost loved ones that day,” Geller said. “It’s not a political event, it’s a human rights event.”

Former 9/11 Commission chairman Lee Hamilton said the U.S. relationship with the Islamic world “is one of the really great foreign policy challenges of the next decades.”

“We’re not going to solve it in a year or two or five or even 10 years. The kind of debates we’re having today in New York City and Florida and other places reflects that. How do we get right, how do we line up this relationship better than we do,” Hamilton said.

New York Police Commissioner Raymond Kelly said officers would guard the mosque site around the clock into next week.

During weekend demonstrations, “we’ll have a significant number of officers on hand to ensure they’re peaceful events,” Kelly said.

One task will be to keep opposing sides separated, he said.

“We don’t want any physical confrontation,” Kelly said, adding that police weren’t anticipating serious trouble.

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Contributing to this report were Associated Press writers Tom Hays, David Caruso and Colleen Long in New York, Ayi Jufridar in Lhokseumawe, Indonesia; Robert Reid in Kabul; Darlene Superville, Anne Flaherty and Lolita C. Baldor in Washington; and AP Legal Affairs Writer Curt Anderson in Miami.