Dozens of Saudis gathered outside the interior ministry in Riyadh on Sunday to demand the release of jailed relatives, an activist said, two days after a planned day of protests fizzled amid a heavy police presence.

Protests are banned in Saudi Arabia and the interior ministry denied one was taking place. Journalists could not get close to the heavily guarded ministry complex.

"They demand the release of relatives in prison," activist Mohammed al-Qahtani told Reuters by phone. He was not part of the gathering.

A call via social media for a day of anti-government protests went largely unheeded on Friday as police stepped up their presence in the capital Riyadh and elsewhere.

Small Shiite protests have taken place in the east.

Saudi Arabia, a key U.S. ally, has so far avoided unrest that toppled rulers in Egypt and Tunisia and spread to other Gulf countries, but dissent has built up in the world's top oil exporter, an absolute monarchy without an elected parliament.

Protests in Riyadh, even small ones, pose a challenge to the Saudi government as it tries to show the country is stable while protests rage over its borders in Bahrain, Yemen and Oman.

Pictures circulating on Twitter showed dozens of men dressed in traditional white robes and red headdresses gathered peaceably outside the ministry. They did not appear to be shouting slogans or holding protest signs.

"There is nothing going on in front of the ministry. I just left the ministry and there was nothing there," Interior Ministry spokesman Mansour al-Turki told Reuters.

Saudi Arabia has guaranteed Western energy supplies for decades, and the calls for protests have put markets on edge.

Amnesty International and other human rights activists have accused Saudi Arabia of having detained a large number of people without trial in its sweep against al Qaeda, which staged a campaign inside the kingdom from 2003-06. Riyadh denies this.