Saudi authorities have stepped up security in Riyadh, where anti-monarchists were expected to try to hold demonstrations called by an exiled opposition leader.
Witnesses say bus loads of additional security forces, some carrying batons, were brought into the city center Thursday, while dozens of police cars patrolled the area.
London-based dissident Saad al-Fagih, who heads the Movement for Islamic Reform, has called for peaceful protest marches in Riyadh and Jeddah after noon prayers Thursday, to demand the monarchy's removal.
Such demonstrations are illegal in Saudi Arabia, and those participating would risk arrest.
Mr. Fagih, has lived in London for eight years. Last year, he accused Saudi agents of attempting to kill him in a stabbing incident at his home. The Saudi government denied any involvement.