The gunmen detonated bombs that sparked a fire and forced the facility to halt operations, officials said. A few hours later, a small refinery in the south shut down after a technical failure sparked a fire in a storage unit, an official said.

If not fixed swiftly, the two shutdowns could translate into long lines at fuel stations and even longer electricity outages.

The dearth of reliable electricity — some Iraqis get just a few hours a day — was one of the leading complaints of protesters during violent anti-government protests across Iraq on Friday.

The attack on Iraq’s largest refinery, Beiji, began at about 3:30 a.m. Assailants carrying pistols fitted with silencers attacked guards and planted bombs near some production units for benzene and kerosene, said the spokesman for Salahuddin province, Mohammed al-Asi.

One guard was killed and another wounded, al-Asi said. He said about 45 soldiers have been brought in temporarily to protect the facility, and that technicians currently repairing the refinery estimated it would be back online later this week.

Iraqi Oil Ministry spokesman Assem Jihad said an investigation would be launched and that he hoped operations could resume shortly.

The Beiji refinery, located about 155 miles (250 kilometers) north of Baghdad, has two sections. The attackers targeted the installation’s North Refinery that handles 150,000 barrels a day. The second section, the Salahuddin Refinery, is under renovation. It used to process 70,000 barrels per day.

At the height of the insurgency from 2004 to late 2007, the Beiji refinery was under control of Sunni militants who used to siphon off crude and petroleum products to finance their operations.

Hours after the Beiji facility was attacked, a small refinery in Samawa, a city on the Euphrates River about 230 miles (370 kilometers) southeast of Baghdad, went offline due to a fire in the storage unit, according to a local official.

The official, who spoke on condition of anonymity because he was not authorized to release information, said the fire was caused by a technical failure, not foul play. He wouldn’t say when work would resume at the plant, which has the capacity of 30,000 barrels a day.

Iraq’s overall refining capacity is currently slightly over 500,000 barrels per day. Its three main oil refineries — Dora, Shuaiba and Beiji — process slightly over half of the 700,000 barrels-per-day capacity they had before the 2003 U.S. invasion.

Iraq has the world’s third-largest known oil reserves with an estimated 115 billion barrels, but its production is far below its potential due to decades of war, U.N. sanctions, lack of foreign investment and insurgent attacks.

Iraq has been importing refined products since 2003 because of the dilapidated refining sector and booming local demand.

Saturday’s closures could spell trouble for Iraqi consumers, especially at a time when the weather is just beginning to warm and more citizens will be relying on their air conditioning.

Also Saturday, health officials and police said two teens, ages 12 and 18, died of injuries sustained in the anti-government protests Friday, bringing the death toll for the day to 14. The officials spoke on condition of anonymity because they were not authorized to release information.

On Friday, thousands marched on government buildings and clashed with security forces in cities across Iraq in an outpouring of anger, the largest and most violent anti-government protests in the country since political unrest began spreading in the Arab world weeks ago.

The protests, billed as a "Day of Rage," were fueled by anger over corruption, chronic unemployment and shoddy public services from the Shiite-dominated government.

Demonstrations continued Saturday in the northern city of Sulaimaniyah 160 miles (260 kilometers) northeast of Baghdad, where nearly 4,000 demonstrators took to the streets. Some protesters threw rocks at Kurdish security forces who threw them back and shot into the air to disperse people.

At least one person was shot and killed and three injured, said the head of the Sulaimaniyah Health Directorate Regald Hama Rasheed.

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Associated Press writers, Hamid Ahmed in Baghdad and Yahya Barzanji in Sulaimaniyah contributed to this report.