It looks like I have been wrong that Asmaa Mahfouz was the initiator of the Egyptian protests on Jan 25th. The NY Times relates the story of how youth protesters from Tunisia and Egypt collaborated for 2 years to start the protests. As their success became evident, the MB joined and others and they stood their ground under assault by the security services of Mubarak, first on Friday, the 28th, the “Day of Rage", where:

Egyptian Hopes Converged in Fight for Cairo Bridge
By the early afternoon, a few thousand protesters faced off against well over a thousand heavily armed riot police officers on the four-lane Kasr al-Nile Bridge in perhaps the most pivotal battle of the revolution.

“We pulled out all the tricks of the game — the Pepsi, the onion, the vinegar,” said Mr. Maher, who wore cardboard and plastic bottles under his sweater, a bike helmet on his head and a barrel-top shield on his arm. “The strategy was the people who were injured would go to the back and other people would replace them,” he said. “We just kept rotating.” After more than five hours of battle, they had finally won — and burned down the empty headquarters of the ruling party on their way to occupy Tahrir Square.

and then again on Feb 2nd:
A Tunisian-Egyptian Link That Shook Arab History
The protesters — trying to stay true to the lessons they had learned from Gandhi, the Rev. Dr. Martin Luther King Jr. and Gene Sharp — tried for a time to avoid retaliating. A row of men stood silent as rocks rained down on them. An older man told a younger one to put down his stick.

But by 3:30 p.m., the battle was joined. A rhythmic din of stones on metal rang out as the protesters beat street lamps and fences to rally their troops.

The Muslim Brotherhood, after sitting out the first day, had reversed itself, issuing an order for all able-bodied men to join the occupation of Tahrir Square. They now took the lead. As a secret, illegal organization, the Brotherhood was accustomed to operating in a disciplined hierarchy. The group’s members helped the protesters divide into teams to organize their defense, several organizers said. One team broke the pavement into rocks, while another ferried the rocks to makeshift barricades along their perimeter and the third defended the front.

“The youth of the Muslim Brotherhood played a really big role,” Mr. Maher said. “But actually so did the soccer fans” of Egypt’s two leading teams. “These are always used to having confrontations with police at the stadiums,” he said.

Soldiers of the Egyptian military, evidently under orders to stay neutral, stood watching from behind the iron gates of the Egyptian Museum as the war of stone missiles and improvised bombs continued for 14 hours until about four in the morning.

Then, unable to break the protesters’ discipline or determination, the Mubarak forces resorted to guns, shooting 45 and killing 2, according to witnesses and doctors interviewed early that morning. The soldiers — perhaps following orders to prevent excessive bloodshed, perhaps acting on their own — finally intervened. They fired their machine guns into the ground and into the air, several witnesses said, scattering the Mubarak forces and leaving the protesters in unmolested control of the square, and by extension, the streets.