A suicide bomber driving a minivan with passengers aboard blew himself up near a bus depot in the center of the capital on Friday, killing seven people and wounding 20, police said.

The charred wreckage of the van smoldered in the hot sun while Iraqi police hurried onlookers away from the scene of the blast in Tayaran Square. The square, a major local bus terminal, was crowded with people enjoying the Friday holiday.

As insurgents have stepped up attacks on crowds of civilians in public places in recent months, bus stations and markets have been frequent targets. A series of car bombings at the main bus station and a nearby hospital in Baghdad in August killed more than 40 people.

Three American soldiers also died Friday, the U.S. military said in statements. Two were killed by roadside bombs, one near Balad, about 50 miles north of Baghdad, and the other in southeastern Baghdad, the statements said. The third soldier was killed by small-arms fire in Ramadi, 60 miles west of the capital.

Meanwhile, in Mosul, 220 miles to the north, police discovered 10 bodies on Thursday night. Three of the dead were identified as members of an ethnically based political party, the Turkmen Front, according to Bahaa Aldin Bakri, a physician at Jumhouri Hospital. The three Turkmen Front members were shot by armed men driving in a red Opel sedan in the Baladiyat neighborhood of eastern Mosul, Bakri said.

In Baghdad on Thursday night, a member of the de-Baathification committee, which vets government employees for ties to Saddam Hussein's outlawed Baath Party, was assassinated, said Ali Lami, a senior committee member. Ali Abdul Ridha, a manager with the committee, was driving with his family in the Nafaq al-Shurta area of the city when he was assassinated by unknown armed men who pulled up alongside his vehicle.

Lami said another member of the committee who had been kidnapped was rescued by Iraqi security forces in the Ghazaliya neighborhood of western Baghdad, a stronghold of insurgents operating in the capital. An Australian hostage, Douglas Wood, was rescued there in June after being held for 47 days.

U.S. and Iraqi officials have warned that violence will likely increase leading up to the Oct. 15 referendum on a new constitution, which will provide the framework for a new government.

On Thursday, Iraq's influential Grand Ayatollah Ali Sistani announced his support for the constitution through an aide in Najaf. The Shiite Muslim leader's endorsement is essential to the success of the referendum, which some Sunni Muslims have vowed to defeat. Some Sunni politicians are upset about elements of the constitution that would create a federal state, preserving for ethnic Kurds in northern Iraq and possibly granting it to the majority Shiite south.

At the Sunni Um al-Qura mosque in Baghdad on Friday, Mahmoud Sumaidaie called for unity, appealing to Muslims in other parts of the Middle East to help stop the violence in Iraq.

"We are all facing these crises, but some are responding while others are complacent and unresponsive," the preacher said. "We want deeds, not words. We need to close our ranks, renounce division."

But Sumaidaie condemned the constitution, which he labeled the "American project here," and called for its defeat. "Our constitution is the Koran," he said. "It's the duty of each one of you to reject this constitution, which is part of the conspiracy targeting this nation."

Shiite worshipers who gathered at the Buratha mosque for Friday prayers said they intended to heed Sistani's call to vote for the constitution.

"I have already read the draft, and I think it fulfills our needs," said Imad Hasan, 32, a government employee. "This constitution will unite Iraq and restore safety back to it. Although many Sunnis would vote against it, millions of Iraqis will support it. I am sure it will succeed."

Asaad Rasem, 30, a merchant, said the constitution would be the key to "real freedom."

"Federalism . . . will unite Iraq and bring safety," he said. "Our religious leaders and the National Assembly members did their best for our interest. Sayyid Sistani is a wise man, and I think he said that for the benefit of all Iraqis."

Special correspondent Dlovan Brwari in Mosul contributed to this report.

U.S. military police, seen through the windows of a charred vehicle, secure the site of a suicide bombing at a local bus terminal in central Baghdad.