Ben Curtis, Associated Press
Egyptian riot police group to push back protesters, unseen, during clashes in downtown Cairo, Egypt, in the early hours of Wednesday, Jan. 26, 2011. Egyptian police fired tear gas and rubber bullets and beat protesters to clear thousands of people from a central Cairo square Wednesday after the biggest demonstrations in years against President Hosni Mubarak's authoritarian rule.

CAIRO — Egyptian activists protested for a third day as social networking sites called for a mass rally in the capital Cairo after Friday prayers, keeping up the momentum of the country's largest anti-government protests in years.

The outlawed Muslim Brotherhood, Egypt's largest and best organized opposition group, has thrown its support behind the demonstrations and if its significant support base joins Friday demonstrations, it will be a big boost to the grassroots movement calling for the ouster of longtime President Hosni Mubarak.

The protesters could also be energized by the imminent return of Mohammed ElBaradei, a Nobel peace laureate and the country's top pro-democracy advocate. ElBaradei is expected to return Thursday night.

In Vienna, ElBaradei told reporters at the airport on his way back to Egypt that "the regime has not been listening." He urged the Egyptian regime to exercise restraint with protesters, saying they have been met with a good deal of violence which could lead to an "explosive situation."

He added that his goal is to make sure things will go in an orderly and peaceful way and says that those who have taken to the streets are expressing their "legitimate need" for an Egypt that is democratic and based on social justice.

The 82-year-old Mubarak, who has led Egypt for nearly 30 years, has not been seen in public or heard from since the protests began Tuesday with tens of thousands marching in Cairo and a string of other cities. The protesters have vented rage over the government's neglect of poverty, unemployment and rising prices.

The demonstrations pose the most serious challenge to date to Mubarak's authoritarian rule and culminate a steady rise in discontent that had already raised serious questions about how long he can keep his grip on power.

Associated Press reporters saw scores of protesters outside the downtown Cairo offices of Egypt's lawyers' union, which has been one of the flashpoints of this week's unrest. About 100 people were also protesting outside police headquarters in the city of Suez east of Cairo, another hot spot.

Meanwhile, social networking sites were abuzz with talk that Friday's rallies could be one of the biggest so far. Millions of Egyptian men gather at mosques across the city for Friday prayers, providing organizers with a huge number of people already out on the streets to tap into.

If Brotherhood supporters turn out in the wake of the group's endorsement of the protest movement, it could swell the numbers of demonstrators significantly. But the group has stopped short of an outright call for its backers to take to the streets.

"We are not pushing this movement, but we are moving with it. We don't wish to lead it but we want to be part of it," said Mohammed Mursi, a senior Brotherhood leader.