Tens of thousands of people gathered in a main square in the Yemeni capital for Friday prayers that later turned into mass protests to press demands for the U.S.-backed president to step down.

Witnesses said the protesters poured into a square near the main gates of Sanaa University for the demonstrations amid tight security.

Security forces set up cordons in nearby streets, trying to thwart protesters from converging in the square that has been a flashpoint for weeks of demonstrations demanding that President Ali Abdullah Saleh, in power for 32 years, step down.

A Muslim imam who led Friday's prayer urged protesters to step up their demonstrations "until the president has departed."

"Every Yemeni is religiously and duty-bound to work to topple this regime," said imam Abdullah Fatir in his sermon, adding that Saleh "is a devil who has driven us to the stone ages." Shouts from the crowd of "Allahu akbar," or "God is great," accompanied his words.

Participants said tens of thousands attended — perhaps one of the largest gatherings since the protests began earlier this month. Violent clashes accompanied protests one week ago — also after Friday prayers — in Sanaa and the southern cities of Aden and Taiz, with several deaths reported.

Some of the demonstrators in the latest protests tried to break the security lines and move to other streets, but police fired into the air and forced them to backtrack.

"We are coming to take you from the presidential palace," activist Tawakul Kermal told the gathering, addressing Saleh.

In Aden, security used tear gas and fired bullets in the air to disperse hundreds of protesters in three neighborhoods of the southern port, and two protesters were wounded. Other demonstrators were staged in Taiz and Hadramout.

On Wednesday, Saleh said he had ordered his security services to protect protesters, stop all clashes and prevent direct confrontation between government supporters and opponents.

Yemen, an impoverished country with a weak central government and an active branch of al-Qaida, has been swept up in the protests inspired by successful uprising in Egypt and Tunisia. Saleh has promised to step down after national elections in 2013, but the demonstrators want him out now.

In recent days, students and other activists have been digging in, setting up encampments in some public areas.

Government forces failed to dislodge the protesters, and thousands more have streamed into the square in support the students, including academics, writers and artists.