Harith al-Ubaidi of the Iraqi Accordance Front said Sunnis were "hand in hand" with Shiites against last week's attack in Karbala, south of Baghdad. His remarks were significant because the Iraqi Accordance Front is the main Sunni coalition that is negotiating with Shiites and Kurds over a coalition government.
"We also demand that the occupier get out, because he is the reason behind every crime," al-Ubaidi said. "If the occupier would leave, Iraqis would live as brothers."
He spoke at the Umm al-Qura mosque, Baghdad headquarters of the Association of Muslim Scholars, a Sunni clerical group that is believed to have ties to some insurgent groups.
The sermon was followed by a demonstration against a U.S. raid on the mosque over the weekend. Hundreds of worshippers took part in the protest. The mosque is in the al-Adel neighborhood, one of Baghdad's roughest.
In other developments:
At dawn Tuesday, mosques in Iraq ushered in the first day of the Islamic festival of Eid. There were no reports of violence as of midday Tuesday.
Many Shiites visit the holy city of Najaf during Eid, but this year some said the trip was too dangerous.
"In spite of the happiness of Eid, we feel very sad that we are not able to visit the holy shrines and the cemeteries because of the deteriorated security situation," said Khadimiya Abbas, 55, a housewife living in eastern Baghdad.
Two insurgents planting a roadside bomb in Samarra were killed Monday when it detonated prematurely, and in two separate incidents in Samarra, U.S. soldiers killed two gunmen that fired on patrols, the military said Tuesday.
Also Monday,in Baghdad and blew themselves up during celebrations of National Police Day, killing 29 Iraqis.
The attackers died before getting near the U.S. ambassador and senior Iraqi officials at the festivities, but the blasts capped a particularly deadly week for American and Iraqi forces.
At least 498 Iraqis and 54 U.S. forces have been killed in violence since the Dec. 15 parliamentary elections.
An Internet site known for publishing extremist material from al Qaeda in Iraq leader Abu Musab al-Zarqawi carried a claim of responsibility for Monday's suicide attack, saying it was in revenge for the torture of Sunni Arab prisoners at two detention facilities run by the Shiite-led Interior Ministry.
"The lions of al Qaeda in Iraq were able to conduct a new raid on the Interior Ministry, taking revenge for Allah's religion and the Sunnis, who are being tortured in the ministry's cellars," the statement said.
The claim, which could not be independently verified, referred to reports that more than 100 abused prisoners were recently found in the jails, bolstering complaints by Sunni Arabs about the treatment of detainees by Interior Ministry forces.
The bombs exploded in quick succession about 1,500 feet from the parade being watched by U.S. Ambassador Zalmay Khalilzad, Interior Minister Bayan Jabr, Defense Minister Sadoun al-Dulaimi and hundreds of others.
None of the officials was hurt and the ceremony was not interrupted, said Lt. Col. Barry Johnson, a U.S. military spokesman. He said the explosions "had no impact on the ceremony and did not require anybody to take cover."