Saudi Arabia has detained 520 suspected al Qaeda-linked militants since January according to the country’s Interior Ministry.
They are suspected of planning car bomb attacks against an oil installation in the kingdom.
A ministry statement read out on Saudi television said the detainees were part of a wider plot managed from abroad.
"Security forces managed to arrest one cell in the Eastern Province led by African residents ... their concern was to get close to people working in the oil sector in order to find work in oil installations," the statement said.
"They planned in fact to attack an oil installation and security target with rigged cars," it added.
The kingdom, which has faced a campaign of violence by al Qaeda-linked militants since 2003, arrested hundreds of suspects in 2007 but because of a tough security crackdown has not faced any major attacks for over two years.
The ministry statement said a total of 701 people had been arrested in recent months but 181 had been released for lack of evidence, leaving 520 in custody.
The last major attack by militants was a failed attempt to storm the world's largest oil processing plant at Abqaiq in the Eastern Province in February 2006.
Saudi Arabia is the world's biggest oil exporter and held a meeting of oil consumers and producers in Jeddah this week in an effort to tame record world oil prices.