Cell phone text messaging was cut late Thursday while Internet services, which had been patchy during the night, were completely severed early Friday, cutting off access to networking sites Facebook and Twitter -- key communications' tools used by organizers of the protests.
Mobile phone services were also disrupted in many areas.
As the unrest swirled, US President Barack Obama warned that violence was not the answer, urging restraint on both sides, and also pressing Mubarak, in power for over 30 years, to adopt political reforms.
Egypt's leading dissident, Nobel laureate ElBaradei, said he would take part in the protests after arriving late Thursday from Vienna.
"It is a critical time in the life of Egypt. I have come to participate with the Egyptian people," ElBaradei, a vocal critic of Mubarak, said after arriving at Cairo airport.
"The desire for change must be respected," he said. "The regime must not use violence in the demonstrations."
He told reporters earlier in Vienna that he was ready to "lead the transition" in Egypt if asked.