Suicide attackers unleashed car bombings Wednesday morning against police buildings in Iraq’s biggest Shiite city, striking rush-hour crowds and killing at least 68 people, including 16 children incinerated in their school buses.

The attacks in Basra wounded about 200 people and marked a revival of devastating suicide bombings, which had not been seen during this month’s battles between U.S. forces and homegrown guerrillas across Iraq.

Basra Gov. Wael Abdul-Latif, a member of the Iraqi Governing Council, blamed al-Qaida, but a U.S. counterterrorism official said it was premature to make such judgments.

In Fallujah, the bloodiest battlefield in April, an agreement aimed at bringing peace to the city ran into trouble Wednesday. Insurgents attacked Marines, prompting fighting that killed 20 guerrillas. Marines said most weapons turned in by residents in a disarmament deal were unusable, undermining a crucial attempt at disarming fighters.

About 350 miles to the south, in Basra, bombers struck just as the city’s main street market, near one of the targeted police stations, was opening for the day. Shoppers were headed to the stalls of vegetables and other goods, and children were passing on their way to school.

The attackers detonated four cars packed with missiles and TNT in front of three police stations — one of them next to Basra’s main street market — and a police academy. An hour later another car bomb went off outside the police academy in Zubair, a largely Sunni town about nine miles from mainly Shiite Basra.

Police discovered two other car bombs before they were detonated and arrested three men in the vehicles.

The last major suicide attack in Iraq also targeted Shiites: Suicide bombers detonated explosives strapped to their bodies among thousands of pilgrims at holy shrines in Karbala and Baghdad on March 2. At least 181 people died.

Elsewhere Wednesday, a suicide attacker bombed a security police building in the Saudi capital, Riyadh, killing at least four people and wounding 148. The attack had the hallmark of an al-Qaida operation, officials said.

The death toll was expected to rise after hospital officials reported early today that body parts have been recovered indicating four more deaths, in addition to the four already confirmed by the Interior Ministry.

In Poland, outgoing Prime Minister Leszek Miller suggested that his country, which commands a division of 9,500 European troops in a violent area of central and southern Iraq, might have to reconsider its commitment to stay in Iraq after Spain’s decision last weekend to withdraw its soldiers. The multinational force that Poland leads includes 2,400 Polish troops.