CAIRO (AP) -- 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 grass roots movement calling for the ouster of longtime President Hosni Mubarak. At least six people have died in clashes, hundreds have been injured and nearly 1,000 detained.
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, who has emerged as a prime challenger to Mubarak's regime, is expected to return tonight.
In Vienna, ElBaradei told reporters at the airport on his way back to Egypt that he was seeking a regime change and was ready to lead the opposition movement.
"The regime has not been listening," he said. "If people, in particular young people, if they want me to lead the transition, I will not let them down. My priority right now ... is to see a new regime and to see a new Egypt through peaceful transition."
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.
Mubarak has not said yet whether he will stand for another six-year term as president in elections this year. He has never appointed a deputy and is thought to be grooming his son Gamal to succeed him despite popular opposition. According to leaked U.S. memos, hereditary succession also does not meet with the approval of the powerful military that provided all of Egypt's four presidents since the monarch was toppled in the 1950s.
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.
In the Suez Canal city of Ismailia, east of Cairo, hundreds of protesters clashed with police who used tear gas and batons to disperse them. There were two other small, peaceful protests by lawyers in the Mediterranean port city of Alexandria and the Nile Delta town of Toukh, north of Cairo.
Meanwhile, social networking sites were abuzz with talk that Friday's rallies could be one of the biggest so far. Millions 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.
Police have used heavy force to suppress demonstrations and the government has banned all gatherings, signaling zero tolerance for any further dissent.
Police have fired rubber bullets, tear gas, and used water cannons to disperse crowds. They have also fired live ammunition in the air at times to warn people and there have been many scenes of riot police in helmets and shields charging crowds and beating people with batons and plainclothes police beating demonstrators with long sticks.
The uncertainty about where the protests are leading and the government's inability to quickly quell them led to a stock market crash today. The benchmark index plummeted more than 10 percent by close, following a 6 percent the day before.
In a statement posted on its website, the Muslim Brotherhood called for protests to remain peaceful. It also called for new parliamentary elections under judicial supervision, the introduction of far reaching reforms and the lifting of emergency laws in force since 1981.
"The movement of the Egyptian people that began January 25 and has been peaceful, mature and civilized must continue against corruption, oppression and injustice until its legitimate demands for reform are met," said the statement.
A spokesman for ElBaradei, Abdul-Rahman Samir, said the former head of the U.N. nuclear watchdog was expected to join protests planned for after the Friday prayers.
ElBaradei 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.
ElBaradei returned to Egypt last year after living abroad for decades and has created a wave of support from reformists. But he so far insisted he would not run in this year's presidential election unless restrictions on who is eligible to contest the vote are lifted and far reaching political reforms are introduced.
ElBaradei also has his detractors, who say he spends too much time away from Egypt and may be lacking a thorough understanding of life here because of the decades he has lived abroad, first as an Egyptian diplomat and later with the United Nations.
But ElBaradei, whose support base is primarily made up of youths, is not tainted by corruption and his international standing would present authorities with a dilemma if he joins protesters.