Saudi police have arrested five suspected insurgents in raids in several localities in the capital and seized large quantities of explosives and money.
Security officials said the five suspects - four Saudis and one foreigner - were believed to be part of a network preparing for a terror attack in Riyadh.
"They were apparently preparing for a terrorist attack, probably soon," one official told The Associated Press on Monday on condition of anonymity because he was not authorised to speak to the media.
Another official said police seized 20kg of explosives and one million Saudi riyals ($373,000) in a raid north of Riyadh. He said police believed the suspects were planning to blow up some government installations.
The officials said police first arrested a suspect who had been under security surveillance. He later informed on the others in other hideouts in Riyadh.
The officials said in at least one raid police exchanged fire with the suspects while they tried to escape their hideout. There was no report of casualties.
The officials did not reveal the nationality of the fifth suspect. He was only identified as "a resident", a reference to expatriates living in the kingdom.
It was not clear if the suspected insurgents were related to al-Qaida in Saudi Arabia, which has launched a series of bloody attacks in recent years in Saudi Arabia.
The kingdom is waging a campaign against Muslim fighters who have staged numerous terror attacks since May 2003, several of them targeting Westerners holding important positions in the oil industry, in an attempt to cripple the economy.
From an initial most wanted list of 26 insurgents issued in December 2003, all but one have been killed or captured.
Saudi police have lately clashed
repeatedly with insurgents
Those on the newer list of 36 are mainly younger, middle-level fighters; 10 of those have been killed.
King Abdullah, who ascended the throne last August after the death of his half brother, Fahd, is determined to push ahead with the crackdown.
Since Abdullah's ascent to the throne, police have carried out raids in Riyadh and the city of Makka, killing six insurgents, including some leaders of al-Qaida in Saudi Arabia.
Al-Qaida seeks to topple the Saudi royal family because of its close ties with the West, particularly the United States.