The authorities said they also seized large amounts of arms and explosives during raids in Riyadh on Friday.
Raids on two hideouts of presumed Islamists netted a booby-trapped car, 21 explosives belts and large amounts of arms and explosives, the interior ministry said.
The raids followed the fatal shooting of six security men and the father of a detained suspect during a search of his home.
The finds were the largest since the interior ministry said on 12 January that it had seized about 300 explosives belts and nearly 24 tons of explosive materials in its hunt for armed opposition activists.
That statement, which coincided with the airing on television of confessions by "members of terror cells", said seizures over the past six months also included more than 300 RPGs and launchers and more than 430 hand grenades.
The latest arrests and weapons seizures come as the kingdom, home to Islam's holiest sites, hosts 1.4 million Muslims who have come from across the world to take part in the annual hajj pilgrimage to Makka.
Saudi authorities say they are
The interior ministry statement said the raids were launched after security men came under a hail of fire from unknown gunmen as they searched the house of Khaled al-Farraj.
The ministry said al-Farraj was "proven" to be linked with the group planning the "terror attack".
Farraj's father was killed along with the six security men during the raid.
The men were buried with with full honors on Friday, with Riyadh Governor Prince Salman bin Abd al-Aziz leading the prayers for the seven dead.
Salman, a brother of King Fahd, told reporters that the six security personnel had not fired a single shot when they unexpectedly came under attack during the house search.
Security forces have repeatedly clashed with suspected Islamist opposition activists in recent months. Several security personnel have been killed in the shootouts.
Hundreds of activists have been rounded up since last May's triple bombings of residential compounds in Riyadh that killed 35 people, including eight Americans.
A car bomb went off at another expatriate housing complex in the Saudi capital in November, killing 17 people, mostly Arabs.
"Violations were perpetuated by the strictly secretive criminal justice system and the prohibition of political parties, trade unions and independent human rights organisations. Hundreds of suspected religious activists and critics of the state were arrested (in 2003), and the legal status of most of those held from previous years remained shrouded in secrecy"
Amnesty International report on Saudi Arabia
Although Saudi Arabia says it is waging a struggle against "terrorism", opposition activists say it is brutally cracking down on justified dissent.
In a recent report on the kingdom, rights group Amnesty International said the Saudi regime had committed "gross human rights violations... exacerbated by the government policy of "combating terrorism".
The report said: "The violations were perpetuated by the strictly secretive criminal justice system and the prohibition of political parties, trade unions and independent human rights organisations.
"Hundreds of suspected religious activists and critics of the state were arrested (in 2003), and the legal status of most of those held from previous years remained shrouded in secrecy."