Two Iraqi women pass by an army tank patrolling during curfew hours today in Baghdad.

BAGHDAD, Iraq — Unidentified gunmen fired mortars and stormed an electricity substation in a southeastern Baghdad suburb, killing at least nine people and injuring three, police said today.

Iraqi police and soldiers, meanwhile, enforced a daytime ban on private vehicles in the strife-prone capital in an effort to avert attacks during midday prayers today, when large numbers of Muslims congregate for the most important service of the week.

The attack on the power substation began when a string of mortar shells slammed into the Nahrwan station on Thursday evening, police Lt. Bilal Ali Majed said. Thirty minutes later, dozens of gunmen arrived and set fire to the facility, he said.

Security guards returned fire, and Iraqi police and army sent in reinforcements, he said.

At least nine people were killed and three injured in the exchanges, police Lt. Mohammed Kheyoun. He identified the victims as guards and technicians employed by the facility. He did not know whether there were casualties among the assailants.

Downtown Baghdad was largely deserted today as the vehicle ban kept most cars off the street. Major shops and gas stations were closed, but small neighborhood groceries opened for their local customers.

Dozens of young boys turned parts of Baghdad's usually busy Saadoun Street into improvised soccer fields, looking clearly unhappy when the odd car disrupted their games.

Prime Minister Ibrahim al-Jaafari warned preachers not to incite hatred or violence in their sermons, threatening them with "severe measures" in a statement Thursday announcing the restrictions.

Residents expressed some frustration that the announcement came only late Thursday, leaving no time to stock up on food and other provisions. But most accepted the move as necessary to blunt a recent surge in sectarian killing and other attacks.

"A curfew is normal in such situations. It's a way to stop terrorists from carrying out their operations," said Saad Juwad Kadim, a taxi driver forced to take today off.

Hundreds have been killed in the violence trigged by the Feb. 22 bombing of a revered Shiite shrine in the central city of Samarra and reprisal attacks against Sunni mosques. An extraordinary daytime curfew and vehicle restrictions last weekend helped curb the worst of the sectarian killing, but attacks continued this week.

Security forces sealed off Baghdad, preventing most vehicles from entering or leaving the city of 7 million, said Capt. Adil Mohan of the traffic police.

Armed police and soldiers in bulletproof vests manned checkpoints across the city, preventing cars and motorcycles from leaving their neighborhoods. Exceptions were made for journalists and families facing health emergencies.

Militiamen loyal to radical Shiite cleric Muqtada al-Sadr were also out in force in the teeming Shiite slum known as Sadr City, helping police check cars and patrol the area.