BAGHDAD, Iraq — More than 10,000 people marched through Baghdad today in support of a national unity government of Sunnis and Shiites, but insurgent attacks killed six Iraqi policemen and two civilians in a fresh surge of violence.

In the Shiite holy city of Karbala, municipal workers doing maintenance work uncovered remains that police believed were part of a mass grave thought to date back to 1991, when Saddam Hussein's regime put down a Shiite uprising in the south.

Today, the remains— discovered on Monday— were sent for testing in an effort to identify the bodies, Karbala police spokesman Rahman Mashawy said. He did not say how many bodies were found and the police claim could not be independently verified.

Human rights organizations estimate that more than 300,000 people, mainly Kurds and Shiite Muslims, were killed and buried in mass graves during Saddam Hussein's 23-year rule, which ended when U.S.-led forces toppled his regime in 2003.

A lull in violence followed the Dec. 15 parliamentary elections. But that ended on Sunday, when 18 people were killed in attacks. On Monday, shootings and bombings killed at least two dozen people, including a U.S. soldier.

Officials said insurgents were trying to deepen the political turmoil surrounding the contested vote. Preliminary figures have given a big lead to the religious Shiite bloc that controls the current interim government.

The new violence came as three opposition groups threatened a wave of protests and civil disobedience if fraud charges are not properly investigated. The warning came from the secular Iraqi National List, headed by former Shiite Prime Minister Ayad Allawi, and two Sunni Arab groups.

More than 10,000 people, some carrying photos of Allawi, demonstrated in Baghdad in favor of a government that would give more power to Sunni Arabs and secular Shiites. Marches chanted "No Sunnis, no Shiites, yes for national unity."

"We're protesting to reject the elections fraud. We want to ask the government and the elections commission: 'Where did our votes go? Who stole them?'" said Abdul Hamid Abdul Razza, a 45-year-old barber.

Clashes erupted between gunmen and Iraqi police in Baghdad, killing two policemen and two bystanders, Capt. Firas Keti said. South of Baghdad, a roadside bomb targeting a police patrol killed two officers, and gunmen in southern Baghdad killed another, police said.

Gunmen southeast of Kirkuk, 180 miles north of Baghdad, killed one police officer and wounded two, Capt. Farhad Talabani said.

The U.S. military today said two U.S. pilots died in a helicopter accident in western Baghdad. The accident was under investigation; the military said no hostile fire was involved.

Iraq's Electoral Commission said Monday that final results for the 275-seat parliament could be released in about a week.

Sunni Arab and secular Shiite factions are demanding that an international body review more than 1,500 complaints, warning they may boycott the new legislature. They also want new elections in some provinces, including Baghdad. The United Nations has rejected an outside review.

"We will resort to peaceful options, including protests, civil disobedience and a boycott of the political process until our demands are met," said Hassan Zaidan al-Lahaibi of the Sunni-dominated Iraqi Front for National Dialogue. He spoke in neighboring Jordan, where representatives of the groups have met in recent days.

The election commission considers 35 of the complaints serious enough to change some local results. But Farid Ayar, a commission official, said there was no reason to cancel the entire election.

He also said preliminary results from early votes by soldiers, hospital patients, prisoners and overseas Iraqis showed a coalition of Kurdish parties and the main Shiite religious bloc each taking about a third. Those nearly 500,000 votes were not expected to alter overall results significantly.