Hundreds of Saudi riot police have launched a crackdown on would-be protesters in the cities of Riyadh and Jeddah, in a show of force triggered by an exiled dissident's call for anti-government demonstrations.

There are reports of arrests in both cities, but authorities have not yet commented. Public protests are banned in the kingdom.

About an hour after the protests were set to begin Thursday, Western news reports say the demonstrations failed to materialize in either city.

Earlier this week, London-based dissident Saad al-Fagih, who heads the Movement for Islamic Reform, called for tens of thousands of Saudis to protest after noon prayers Thursday against the Saudi monarchy.

Last year, he accused Saudi agents of attempting to kill him in a stabbing incident at his London home. The Saudi government denied any involvement.

Some information for this report provided by AP and Reuters.