Security forces in Egypt are on high alert, with thousands of people expected to join anti-government rallies after Friday prayers.
The government says it is open to dialogue but also warned of "decisive measures" as the fourth day of violent protests loomed.
At least seven people have died and up to 1000 have been arrested since the protests on Tuesday.
There has been widespread disruption to internet and mobile phone services. There were reports of fresh clashes overnight on Thursday, as well as opposition figures being arrested, the BBC reports.
The apparent crackdown on the largest opposition movement, the Muslim Brotherhood, came after it said it would back the protests on Friday.
Egyptian opposition figure and Nobel Peace Prize laureate Mohamed ElBaradei returned to Cairo on Friday after years working overseas and promised to join the demonstrators.
Mr ElBaradei says Egypt must change and he is ready to lead the transition if asked.
The 68-year-old former head of the International Atomic Energy Agency says the people's fear has been broken and nothing will stop them now.
He warned that if the Egyptian government used force, it would lead to more violence.
"I continue to call on the regime to understand that they better listen and listen quickly, not use violence and understand that change has to come. There's no other option."
President Hosni Mubarak has not been seen in public since Tuesday's "day of revolt", inspired by the uprising in Tunisia which ousted President Zine al-Abidine Ben Ali on 14 January.
The Egyptian government tolerates little dissent and demonstrations are outlawed.
However, protesters have defied the ban and have remained on the streets of Cairo and other cities, including the northern town of Suez where protesters clashed with police on Thursday.