Key communication tools used by protest organizers have been severed.
Egypt cut mobile phone and Internet services on Friday.
The move cut off access to networking sites Facebook and Twitter, key communication tools used by protest organizers.
Egypt cut mobile phone and Internet services on Friday and sent columns of riot police trucks into Cairo in a bid to thwart thousands of activists due to join anti-regime protests after noon prayers.
Leading dissident Mohamed ElBaradei and the Muslim Brothers have joined the biggest uprising in decades despite the government warning that decisive measures would be taken to crush the rising tide of protest.
Streets around Cairo were quiet Friday, a weekly holiday in Egypt, as dozens of riot police trucks were seen headed towards the capital, an AFP reporter said.
Egyptian police have struggled to deal with protesters, who for the past three days have clashed heavily with police in Cairo and other cities as they demand the ouster of veteran President Hosni Mubarak.
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.
The country's largest opposition movement, the Muslim Brotherhood, said that it too would participate in Friday's protests, which are due to start after the noon prayers as thousands of faithful emerge from mosques across the country.
At least 20 members of the Muslim Brotherhood were arrested overnight Friday, including five former members of parliament, the group's lawyer Abdelmoneim Abdel Maqsoud told AFP.
The nationwide demonstrations, inspired by the "Jasmine Revolution" in Tunisia, have swelled into the largest uprising in three decades, sending shockwaves across the region.
Seven people have been killed -- five protesters and two policemen -- and more than 100 injured.
A security official told AFP around 1,000 people had been arrested since the protests began.
Human Rights Watch said eight demonstrators and a policeman had been killed in protests in Cairo, Suez, Alexandria and other cities.
The US-based group said Egyptian police had escalated the use of force against largely peaceful demonstrations, calling it "wholly unacceptable and disproportionate."
Clashes erupted in the cities of Suez and Ismailiya on Thursday, and in a Sinai town where police shot dead a protester, witnesses said.
Members of the pro-democracy youth group April 6 Movement have vowed to continue to take to the streets, defying a ban on demonstrations announced on Wednesday.
Activists had circulated SMS messages and posted appeals on Facebook for fresh demonstrations "to demand the right to live with freedom and dignity."
Obama, in his first on-camera reaction to the demonstrations, said "violence is not the answer in solving these problems in Egypt" and that it was "absolutely critical" for Mubarak to move towards political reform.
Egypt is one of the United States' closest allies in the region, but analysts say Washington is growing increasingly concerned that its refusal to implement more political reforms could lead to further unrest and instability.
Among protesters' demands are the departure of the interior minister, whose security forces have been accused of heavy-handedness, and an end to a decades-old state of emergency and a rise in minimum wages.
Political discontent has been rumbling more loudly in Egypt since parliamentary elections in November, which were widely seen as rigged to allow candidates from Mubarak's ruling National Democratic Party to record a landslide victory.