A car bomb attack in Riyadh has killed four people, including two security officials, Saudi Arabia says.
The blast wrecked the security forces headquarters in Riyadh
The suicide attack also wounded 148 people, the interior ministry said in a statement.
A witness said he saw a car drive up to a protective barrier outside the headquarters of the security forces in the centre of the city.
Just after guards stopped the vehicle, it exploded, sending thick smoke high into the air.
Police have repeatedly clashed with Islamic militants around Riyadh in the last fortnight and officials say they have intercepted five vehicles packed with explosives.
Last week the United States ordered all non-essential diplomatic staff to leave Riyadh.
The Saudi authorities have denounced the attack as the work of terrorists and say they have found the body of a suicide bomber.
The attack happened at 1400 local time (1100 GMT) near the General Security building on al-Washm Street in Riyadh's central Nasiriya district.
Television pictures showed that the explosion had caused widespread damage to the facade of the building and destroyed a number of cars.
Fight for survival
Crown Prince Abdullah, the effective ruler of Saudi Arabia, said at last Monday's cabinet meeting that terrorism would be confronted and every citizen was now a member of the security forces.
BBC Middle East correspondent Paul Wood says the Saudi royals seem now to have realised they are in a fight for their very survival as rulers of the state which carries their name.
Riyadh suffered two major suicide bombings in 2003. A total of 51 people were killed in the attacks in May and November, including the bombers themselves.