A spokesman for Sadr, Salman al-Foureji, said the protesters accused Prime Minister Nouri al-Maliki of collaborating "with the occupation forces," Voices of Iraq said.
Meanwhile, U.S. military officials said the conflict between Shiite militias, including the Mahdi forces, and Iraqi security forces could escalate in the coming weeks but stressed the U.S. military would act only in a supporting role, The Washington Times said Thursday.
Sadr's backers accuse the U.S. military and Iraqi government of taking advantage of the extended cease-fire to liquidate its members.
Sadr called for an extension of a six-month cease-fire on March 1, promising those that violated the agreement would be disowned by the Sadrists. Sadr said, however, that forces could respond to attacks by U.S. and Iraqi forces.
U.S. officials, the Times said, believe Sadr's forces are splintering and lack general cohesion, noting Sadr lacks control over all elements of his security forces.
Other analysts questioned whether the attacks in Basra were coordinated with other outbreaks of violence in Baghdad and other Shiite strongholds and wonder if the latest conflicts amount to a Shiite uprising, the BBC said Thursday.
Regardless, the conflict in Basra represents a test of the capabilities of the Maliki government, as U.S. military officials note the Basra operation is "completely" an Iraqi mission.
Pentagon press secretary Geoff Morrell, however, classified the conflict in Basra as "a byproduct of the success of the (U.S. troop) surge," noting the ability of the Iraqi government to act independently against aggression.