Riyadh, Saudi Arabia ? Saudi Arabia announced Friday that an anti-terrorism sweep netted 172 Islamic extremists and stopped plans to mount air attacks on the kingdom’s oil refineries, break militants out of jail and send suicide attackers to kill government officials.

An official said the plotters had completed preparations for their attacks, and all that remained to put the plot in motion “was to set the zero hour.”

It was one of the biggest roundups since Saudi leaders began cracking down on religious extremists four years ago after militants attacked foreigners and others involved in the country’s oil industry seeking to topple the monarchy for its alliance with the U.S.

But while the monthslong police operation provided a high-profile victory for the royal family, the large number of people arrested highlighted the extremism threat in the world’s leading oil exporter.

The Interior Ministry said the plotters were organized into seven cells and planned to stage suicide attacks on “public figures, oil facilities, refineries : and military zones,” including some outside the kingdom. It did not identify any of the targets.

The militants also planned to storm Saudi prisons to free jailed militants, the ministry’s statement said.

“They had reached an advance stage of readiness, and what remained only was to set the zero hour for their attacks,” the ministry’s spokesman, Brig. Mansour al-Turki, said in a phone call. “They had the personnel, the money, the arms. Almost all the elements for terror attacks were complete.”

The Saudi statement said some of the detainees had been “sent to other countries to study flying in preparation for using them to carry out terrorist attacks inside the kingdom.”

Al-Turki said he didn’t know whether the militants who trained as pilots planned to fly suicide missions like those in the Sept. 11 attack on the United States or whether they intended to strike oil targets in some other way with the aircraft.

“I have no information on what they were planning to do with the airplanes, but I assume, based on the possible use of airplanes in attacks, that they planned to fly the airplanes into specific targets,” he said.

The militants were detained in successive waves, with one group confessing and leading security officials to another group as well as caches of weapons, al-Turki said. He told the privately owned Al-Arabiya television channel that some of those arrested were not Saudis.