Egyptian security forces 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.
There has been widespread disruption to internet and mobile phone services.
There were reports of fresh clashes overnight, as well as opposition figures being arrested..
The apparent 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 demonstrators.
At least seven people have died and up to 1,000 have been arrested since the protests began. They follow the so-called Jasmine Revolution in Tunisia, which saw President Zine al-Abidine Ben Ali flee into exile.
Friday’s rallies in Egypt are expected to be the biggest so far; people have been urged via internet sites to join after attending prayers.
The organisers called on people to come out in force, stressing that the religion of protesters was not relevant.
However, most internet and mobile-phone data connections appeared to be cut off from early on Friday. Service providers have given no reasons.
On its website, the Muslim Brotherhood accused the government of engineering the disruption “to prevent the voices of the Egyptian people from reaching the world”.
The Associated Press news agency reported that the elite counter-terrorism force had been deployed to key locations in Cairo, including Tahrir Square, where earlier protests were held.
But Egyptian film-maker Ahmed Rasheed, who is planning to take part in Friday’s demonstrations, says people no longer fear arrest.
“We have broken this fear barrier,” he told the BBC. “People are taking to the streets, young people, all walks of life, educated, non-educated, higher social classes, lower social classes.”
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.
A security source told Reuters news agency: “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.
The BBC understands security chiefs are telling President Mubarak that they can handle any trouble which might happen after Friday prayers.
Mr Mubarak, 82, has been in office since 1981.
The Egyptian government tolerates little dissent and opposition demonstrations are routinely outlawed.
On Thursday Mr Mubarak’s National Democratic Party said it was ready for dialogue but did not offer any concessions.
Safwat el-Sherif, the party’s secretary general, said: “The NDP is ready for a dialogue with the public, youth and legal parties. But democracy has its rules and process. The minority does not force its will on the majority.”
Returning to Cairo on Thursday, Mr ElBaradei said: “I wish we did not have to go out on the streets to press the regime to act.”
He called on the government to “listen quickly, not use violence and understand that change has to come. There’s no other option.”
Thursday saw protests in Cairo and other areas. In the Sinai region, a Bedouin man was shot dead by security forces.
In Suez, police fired rubber-coated bullets and used tear gas and water cannon, witnesses said. A fire station was set alight by demonstrators.
The unrest in Suez continued late into the night. One protester in the city told Reuters: “This is a revolution. Everyday we’re coming back here.”
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.