After days of protests in the Arab world that have toppled one president and shaken many others, demonstrators calling for the ouster of President Hosni Mubarak poured from mosques in Cairo after noon prayers on Friday, clashing with police who fired tear-gas and water-cannons, according to news reports and images broadcast on television.

Protests were also reported in the cities of Suez, Alexandria and several others, including Al Arish in northern Sinai.

According to The Associated Press, police doused one of the most prominent opposition figures, Mohammed ElBaradei, with a water-cannon and beat supporters who tried to shield him.

Mr. ElBaradei, the former head of the International Atomic Energy Agency, returned to Cairo on January 27, promising to join the largely leaderless protests that have so far been propelled by young people.

Also on January 27, thousands of protesters took to the streets of Yemen, one of the Middle East’s most impoverished countries, demanding the ouster of the 32-year-old American-backed government of Ali Abdullah Saleh, vowing to continue until the government either fell or consented to reforms, The New York Times reported.