Insurgents staged attacks against U.S. forces, schools to be used as polling stations and political party offices on Wednesday, as they pressed a bloody campaign to undermine Iraq's weekend elections. A U.S. Marine transport helicopter crashed in western Iraq, killing 31.

Three car bombs exploded Wednesday in Riyadh, a tense town north of Baghdad, killing at least five people, including three policemen. One of the car bombs targeted a U.S. convoy but there was no report of casualties, police said.

In Baghdad's Sadr City (search) district, Iraqi forces backed by U.S. troops raided a Shiite mosque, detaining up to 25 followers of a radical cleric Muqtada al-Sadr, police and the cleric's supporters said.

A U.S. Marine helicopter transporting troops crashed Wednesday in the desert of western Iraq, killing 31 people, American military officials said. It was the deadliest crash of a U.S. military helicopter in Iraq.

A Pentagon source said the helicopter was a CH-53 Sea Stallion, which is normally configured to carry 37 passengers, but can take up to 55. There was no immediate word on how many people were on board or what caused the crash.

Meanwhile, a U.S. convoy was attacked on the dangerous road to Baghdad airport and at least one vehicle was destroyed, witnesses said. The U.S. and British embassies banned their staffs from traveling on the road last year because of repeated attacks on the highway. There was no word on casualties.

U.S. troops found at least six bombs at different locations around Baghdad, the military said. Iraqi police discovered two more bombs in the Shiite holy city of Najaf (search), where turnout in the Sunday national elections is expected to be high.

In a statement, the U.S. command said the six bombs were discovered early Wednesday in widely scattered areas of the Iraqi capital.

"We've been very successful finding and destroying improvised explosive devices in Baghdad, limiting the insurgent's ability to kill or injure innocent Iraqis," said Maj. Philip Smith, a spokesman for the 1st Cavalry Division and Task Force Baghdad.

The car bombs in Riyadh, located about 40 miles southwest of Kirkuk, exploded at a police station, in front of the mayor's office and along a road used by U.S. troops, police said. Nine people were injured in addition to the five deaths.

Residents of the insurgent-filled city of Ramadi, 70 miles west of Baghdad, reported clashes there Wednesday between U.S. troops and rebels. The fighting erupted when militants attacked a U.S. patrol with rocket-propelled grenades, the residents said. One Iraqi was killed and two were wounded, doctors said.

Insurgents also attacked buildings linked to Sunday's national elections. The rebels have threatened attacks against polling centers, candidates and voters in an attempt to derail the vote.

Two schools slated to be used as polling stations were bombed overnight, and a bomb was found in a third school but defused.

A ground floor classroom in one of the buildings, a preparatory school for girls, was littered with shattered glass and its main entrance was blackened and clogged with debris.

Al-Arabiya television broadcast a videotape showing three men identified by insurgents as election workers who were kidnapped in the northern city of Mosul. The satellite station said the three were abducted by the Nineveh Mujahedeen, which threatened to attack polling stations on election day.

The Iraqi government on Wednesday announced it would ban travel between provinces and extend the hours of curfew from 7 p.m. until 6 a.m., starting Friday, as part of heightened security before the elections.

Also Wednesday, a 1st Infantry Division soldier was killed and two others were wounded when insurgents attacked their patrol with rocket propelled grenades near the northern town of Duluiyah, the U.S. command said.

In other election-related attacks, gunmen opened fire on the local headquarters of the Communist Party and a major Kurdish party north of Baghdad on Wednesday, a police official said.

Assailants blasted the two buildings with heavy machine gun fire and also shot dead a traffic policeman in the city of Baqouba, said police 1st Lt. Hassan Ahmed. The buildings house the city's offices of the communists and the Patriotic Union of Kurdistan (search). There was no word on casualties in those shootings.

In Sadr City, Iraqi forces backed by American troops raided the Al-Rasoul mosque in the teeming Shiite quarter, where U.S. troops battled followers of cleric al-Sadr last year until a peace agreement was struck.

Police said the raid took place occurred shortly before sunset Tuesday. Officials gave no reason for the raid and it was unclear how many worshippers remained in custody Wednesday.

An al-Sadr aide, Abdul-Hadi al-Daraji, denounced the raid as a "criminal act."

"This move by U.S. soldiers aims at provoking the al-Sadr movement in order to accuse us of sabotaging the elections and this is an ugly maneuver," al-Daraji said.

Al-Sadr has declined to run in the Sunday national elections, but some of his followers are running on tickets led by interim Prime Minister Ayad Allawi (search) and one endorsed by the leading Shiite cleric, Grand Ayatollah Ali al-Sistani (search).

Iraqis will choose a 275-member National Assembly and regional legislatures. Sunni Muslim extremists have threatened to sabotage the election and many Sunni clerics have called for a boycott because of the presence of 170,000 U.S. and other foreign troops.

On Tuesday, a video showed an American kidnapped in November pleading for his life as the hostage-takers pointed a rifle at his head.

In the video, a bearded Roy Hallums (search), 56, said he had been taken by a "resistance group" because "I have worked with American forces." He appealed to Arab leaders, including Libya's Moammar Gadhafi, to save his life.

Hallums was seized by gunmen Nov. 1 along with Robert Tarongoy (search) of the Philippines at their compound in Baghdad's Mansour district. The two worked for a Saudi company that does catering for the Iraqi army. The Filipino was not shown in the video.