Mr. Maliki has been under considerable pressure, particularly from American officials and Sunni Arab leaders, to rid the country’s Shiite-run security and intelligence forces of militia influence and human rights abuses. American officials have warned Iraqi leaders that they might curtail aid to the Interior Ministry, which runs the country’s police forces, if officials who commit “gross violations of human rights” and are not held accountable.

Image More than 1,100 American and Iraqi troops moved into Sadr City. Credit... The New York Times

The Interior Ministry, dominated by Shiites, has long been accused by Sunni Arabs of complicity in torture and killings.

It was unclear whether the Basra detainees were still in custody, and there was no further information on the of the detainees or their captors. A spokesman for the British military command in Basra, Maj. David Gell, said in a telephone interview early Monday that he could not provide any further information about them. Reuters news agency said the detainees included a woman and two children.

The raid in Basra comes as the British command is planning a drawdown of as many as 1,600 of its 7,200 troops, a decision driven by the British government’s assessment that Iraqi forces are strong enough to manage the security of the region on their own. In announcing the reduction on Feb. 22, Prime Minister Tony Blair said that while the Basra area remained dangerous, British troops faced far less violence than Americans farther north and that “the next chapter in Basra’s history can be written by Iraqis.”

It was unclear whether the discovery in Basra would have any impact on British planning.

Major Gell said the intelligence agency office was raided after an investigation earlier Sunday led to the capture of five suspected bomb makers and evidence that pointed to possible violations at the agency’s offices. In those offices, he said, “Evidence of significant criminal activity such as torture was found.”

In Baghdad, Iraqi and American soldiers began to patrol the streets of Sadr City and conducted house-to-house searches for illegal weapons and militia fighters linked to sectarian crimes, according to residents and the American military command. No shots were fired.