Saudi TV reported that police sealed off streets near the sight of the explosions and sent helicopters to search for the suspects in Wednesday's apparently coordinated strikes on the kingdom's security apparatus.
An Interior Ministry statement blamed a "deviant group" — the government's term for al Qaeda — for the attacks, which began at about 8:35 p.m. in central Riyadh near the Interior Ministry, which is in charge of Saudi security forces.
A police official said two militants detonated a car bomb by remote control in a traffic tunnel near the ministry. But Al Riyadh, a state-controlled newspaper, said the attack was a suicide bombing, quoting a spokesman for the Interior Ministry, Brig. Gen. Mansour al-Turki.
Saudi TV said a bystander, a limousine driver, was killed. The ministry reported no deaths but said five security agents and a few bystanders were injured, none seriously.
A half hour after the first blast and five miles away, a second explosion went off at a center for recruiting security troops. Police said two suicide bombers tried to storm the center but blew up their car prematurely after police fired on them.
The Interior Ministry said 12 security officers and an unspecified number of bystanders were slightly injured.
Three attackers were killed at the sites of the blasts, according to media reports, and the ministry said seven others were killed after a chase to the north of the capital.
Al Riyadh said more than 90 people were injured in the attacks, mostly security forces and bystanders.
"This is a heinous and disturbing crime," Prince Ahmed bin Abdel Aziz, the deputy interior minister, told Saudi TV. He said the attackers were all Saudis and described them as "terrorists (who) took a great risk because they know that their end is imminent."
The Saudi government has been cracking down on militants since a series of al Qaeda-affiliated attacks began in May 2003. The attacks have killed scores of Westerners.
The attack on the ministry shattered windows in that building and others nearby. The Civil Service Ministry, a post office, a sports club, and a luxury hotel are in the area, known as al-Murabaa.
The Interior Ministry, a massive, modern high-rise, had few signs of damage in pictures broadcast by Saudi television. Damaged cars, including a blood-splattered taxi, sat outside.
The bombings came late in the evening, when few people would have been in the building. But Wednesday night was the beginning of the weekend in the kingdom, so streets were crowded with cars, and civilians.
"I thought first thing they were fireworks," one witness to the first blast told Saudi TV. Another said many at the sports club were injured.
By attacking at night and focusing on targets associated with Saudi security forces, the militants appeared to be trying to avoid criticism they do not value Arab and Muslim lives. That would mark a shift from previous attacks, which seemed designed to maximize casualties, often Arab and Muslim.
Oil prices rose more than 4 percent Wednesday in New York, with analysts citing fears of instability in Saudi Arabia, which has the world's largest oil reserves.
Andrew Mitchell, a spokesman for the U.S. embassy in Riyadh, issued a message advising Americans in Saudi Arabia to "be aware of their surroundings, exercise caution, and monitor news reports closely."
Osama bin Laden's tape earlier this month was his first message directed specifically at Saudis in years, and it was issued 10 days after militants attacked the U.S. consulate in Jiddah, killing nine people.
Bin Laden praised those who carried out the consulate attack and urged his followers to attack the kingdom's oil installations to weaken both the West and the Saudi royal family. Bin Laden accuses the West, particularly the United States, of seeking to destroy Islam and criticizes the Saudi royal family for its alliance with Washington.