BAGHDAD,(Reuters) – Iraq’s government imposed a one-day curfew on the capital Baghdad on Saturday without explanation, ordering all cars and pedestrians off the streets.

The U.S. military said it had arrested a man at the home of the leader of the main Sunni political bloc on suspicion of planning a series of car bomb attacks on the Green Zone, the vast government and diplomatic compound in the city centre.

“Coalition Force personnel detained an individual at the residence of Dr. Adnan al-Dulaimi in Baghdad Sept. 29. The detained individual is suspected of involvement in the planning of a multi-vehicle suicide operation inside Baghdad’s International Zone,” the military said in a statement.

It said the man may have been linked to al Qaeda, and the plan might have been to use suicide vests to attack the Green Zone. U.S. forces did not enter Dulaimi’s house, but searched a security trailer there and the suspect’s car, it said.

Dulaimi is the leader of the Iraqi National Accordance front, the largest Sunni Muslim political bloc in parliament.

Although the sudden imposition of the curfew was not officially tied to the arrest, the curfew was announced on state television late on Friday an hour after a report on the raid at Dulaimi’s house, suggesting there may have been a link.

Iraq has seen a surge in violence this week with the start of the holy month of Ramadan. Sectarian killings have risen sharply and continuously since February, with more than 6,500 people slain in the last two months, according to U.N. data.

As dawn broke, streets in the centre of the capital were quiet. U.S. helicopters periodically flew overhead.

The curfew would remain in place until 6:00 a.m. (0200 GMT) on Sunday, Prime Minister Nouri al-Maliki’s office said. The U.S. military said the curfew was the Iraqi government’s decision, and such measures were effective in the past.

The massive surge in sectarian killings since February has been marked by dozens of corpses being found nearly every day dumped in the streets of Baghdad, bound, tortured and shot.

Sunni Arabs say some of the killings are carried out by Shi’ite death squads with links to the government and police. Increasingly, U.S. officials have backed up such claims.

One senior U.S. military official this week said police had allowed death squads to re-enter areas already cleared by U.S. forces in a seven-week-old crackdown in the capital.

Washington’s ambassador to Iraq threatened to cut off funding for the Iraqi police if the government does not punish police officials for torture and human rights violations.

Zalmay Khalilzad said in an interview with the New York Times that he had faith in the motives of Iraq’s new Interior Minister Jawad al-Bolani, in charge of the police since June.

But he said U.S. officials were reviewing programmes under a law named for Democratic Senator Patrick Leahy that bans U.S. funding for armies and police forces that violate human rights.

“There is a Leahy Law that affects support if the terms of the law are not observed and implemented, and he (Bolani) has assured us that he will do so,” Khalilzad said.

Outside Baghdad, a suicide car bomb targeting an Iraqi army checkpoint in the northern town of Tal Afar killed two people and wounded 30. Other bombs struck in Mosul and Kirkuk in the north and in Iskanderiya south of Baghdad.

In Washington, where Iraq has become a crucial political issue ahead of a congressional election in November, the U.S. Congress voted to block the Bush administration from building permanent bases in Iraq or taking control of its oil sector.

Those provisions were contained in a bill which authorised $70 billion to pay for the wars in Iraq and Afghanistan through the middle of the next fiscal year. Congress has now approved about $507 billion for the two wars, most spent in Iraq.

The Baghdad curfew came at the end of a week of clashes and bombings which began with the start of Ramadan. U.S. commanders say the week saw a record number of suicide bombings.

On Friday gunmen killed the brother-in-law of the chief judge in former leader Saddam Hussein’s genocide trial and badly wounded his sister and nephew.

It was at least the fourth killing closely connected to the U.S.-sponsored court, raising questions about Iraq’s ability to conduct fair trials in a nation on the verge of civil war.