The government says it is open to dialogue but also warned of "decisive measures" as the fourth day of violent protests loomed.
Widespread disruption has been reported to the internet and mobile phone messaging services.
There are also reports of arrests of opposition figures overnight.
The reported crackdown on the largest opposition movement, the Muslim Brotherhood, came after it said it would back the Friday protests.
On Thursday, Egyptian opposition figure and Nobel laureate Mohamed ElBaradei returned to Cairo, promising to join the street protests.
At least seven people have died since the protests began on Tuesday. They follow the so-called "Jasmine Revolution" in Tunisia, which saw President Zine al-Abidine Ben Ali flee into exile.
'Security sweeps'
Friday's rallies in Egypt are expected to be the biggest so far, with people being urged via internet sites to join the protests after attending prayers.
The protest organisers have called on people to come out in force, stressing that the religion of protesters is not relevant.
Late on Thursday, the Facebook and Twitter social websites had been disrupted along with mobile phone messaging, followed by loss of many internet services.
One internet user in Cairo, who wanted to remain anonymous, told the BBC the 3G was not working and SMS messages were not being received.
He said: "Tomorrow's protest may exceed Tuesday's numbers, and I think tension will be high. People are trying to stay safe and are moving around in groups."
Associated Press news agency reported that the elite special operations counterterrorism force, which is rarely seen on the streets, had been deployed to key locations in Cairo, including Tahrir Square, where earlier protests have been held.
Egypt's interior ministry has warned it will take "decisive measures" against the protesters.
A lawyer for the Muslim Brotherhood told the BBC that tens of its members had been arrested.
Reuters news agency quoted a security source as confirming the authorities had ordered a crackdown. The source was quoted as saying: "We have orders for security sweeps of the Brotherhood."
Despite an official ban, the Muslim Brotherhood remains Egypt's largest and most organised opposition movement.
'No Option
On Thursday, Mr ElBaradei arrived in Cairo and said he would join the protests.
Suez was among the cities that saw protests on Thursday
"I wish we did not have to go out on the streets to press the regime to act," he said.
Mr ElBaradei called on the government to "listen quickly, not use violence and understand that change has to come. There's no other option."
The US government, which counts Egypt as one of its most important allies in the Arab world, has so far been cautious in expressing support for either side.
President Barack Obama described the protests as the result of "pent-up frustrations", saying he had frequently pressed Mr Mubarak to enact reforms.
He urged both sides not to resort to violence.
On Thursday there were protests in Cairo, Suez and Ismailiya, while in the Sinai region, a young Bedouin man was shot dead by security forces.
Up to 1,000 people have been arrested.
In Suez, police fired rubber-coated bullets, tear gas and water cannon, witnesses said. A fire station was set alight by demonstrators.
One protester in the city told Reuters: "This is a revolution. Everyday we're coming back here."
