Iraqi Shiites carry images of radical anti-American cleric Muqtada al-Sadr in the Shiite enclave of Sadr City in Baghdad. Thousands protested the second bombing of a Shiite holy shrine Askariya or Golden Dome in Samarra on Wednesday, and Baghdad was under a strict curfew in an attempt to keep a lid on sectarian reprisals.

Baghdad, Iraq ? A citywide clampdown emptied Baghdad’s streets of all vehicles Thursday in attempts to hold off what authorities dread: a storm of Shiite attacks in revenge for the bombing of one of their main shrines. The tactic appeared to keep a lid on widespread violence, but extremists fired shells into the city’s protected Green Zone during a visit by the State Department’s No. 2 official.

The barrage of rockets and mortars included one that hit on a street close to the Iraq parliament less than a half hour before Deputy Secretary of State John Negroponte passed nearby.

The attack again showed militants’ resilience – including their ability to strike the heavily protected zone – despite a U.S.-led security crackdown across the city that began exactly four months ago. But officials paid much closer attention to any signs that Shiites could unleash another wave of retaliation against Sunnis for the Wednesday blasts at the Askariya mosque compound in Samarra.

The first attack on the site in February 2006 sent the country into a tailspin of sectarian violence that destroyed Washington’s hopes of a steady withdrawal from Iraq. On Wednesday, bombers toppled the two minarets that stood over the ruins of the mosques famous Golden Dome about 60 miles north of Baghdad.

The U.S. ambassador to Iraq, Ryan Crocker, echoed Washington’s claim that the latest attack was the work of al-Qaida.

“I just don’t think there’s any doubt that it was al-Qaida that first struck the Askariya in February 2006, and the method this time was very similar to that – (explosive) charges very carefully placed to devastating effect,” Crocker told a group of reporters.

Negroponte called the Samarra attack a “deliberate attempt by al-Qaida to sow dissent and inflame sectarian strife among the people of Iraq.”

The U.S. military issued a statement Thursday saying Iraqi forces had arrested the commander and 12 policemen responsible for security at the shrine, which holds the tombs of two revered 9th century Shiite imams. It was not immediately clear whether the police arrested are suspects in the attack or held for questioning.

Curfews and increased troop levels appeared to hold down retaliatory attacks. The vehicle ban was expected to last through Saturday.

But it did not fully prevent Shiite anger from turning violent.

Four Sunni mosques near Baghdad were attacked or burned within several hours of the Samarra bombings, police said.

Police in the southern city of Basra said Thursday that four people were killed and six wounded in attacks on at least four mosques on Wednesday.