|Clashes between protesters and police erupted outside a mosque in Cairo [AFP]
Protests have erupted in cities across Egypt, with demonstrators demanding an end to Hosni Mubarak's presidency.
Tens of thousands took to the streets across the country following Friday midday prayers.
State TV said a curfew will be imposed from 6pm to 7am local time.
Egyptian military vehicles were sighted on the streets of Cairo on Friday after a day of violent clashes between police and protesters demanding an end to President Hosni Mubarak's rule.
Protesters had previously chanted slogans calling for the army to support them, complaining of police violence during clashes on Friday in which security forces fired teargas and rubber bullets.
"Where is the army? Come and see what the police is doing to us. We want the army. We want the army," the protesters in one area of central Cairo shouted, shortly before police fired teargas on them.
Al Jazeera's Ayman Mohyeldin, reporting from Cairo, said protesters reacted positively when an armoured vehicle with troops showed up, possibly indicating that it belonged to the military.
"The army is a respected establishment in Egypt, and many feel they need their support against what they see as excessive force by the police and security forces," he said.
Earlier, clashes between protesters and police erupted outside a mosque in Cairo.
Protesters reportedly threw stones and dirt at the police after security forces confronted them. They held up posters saying "No to dictatorship" and stamped on posters of Mubarak.
Al Jazeera's Rawya Rageh, reporting from the Mediterranean port city of Alexandria, said protesters streamed out of mosques shortly after prayers to chant slogans against Mubarak. Police responded immediately, firing tear gas to disperse the crowd.
Alexandria is a stronghold of the Muslim Brotherhood, Egypt's technically banned but largest political opposition group, but Rageh said the crowds in the city predominantly consisted of "ordinary citizens".
"This is the same mosque where protests were held against police brutality in June after a 20-year-old man was beaten to death by police," she said. "It’s very symbolic that the current protests are taking place at the same place all over again."
Protests were also reported in Suez, a port on the Red Sea east of Cairo, and in the Nile Delta cities of Mansoura and Sharqiya, witnesses said.
Friday marked the fourth consecutive day of protests in the Middle East's most populous nation coming on the heels of a social uprising in nearby Tunisia that ousted that country's president of 23 years.
Mohamed ElBaradei, the former head of the United Nations' nuclear watchdog and an opposition leader in Egypt, returned to the country on Thursday night after telling reporters he was ready to lead a "transition" if asked. On Friday, he prayed with thousands of worshippers at a mosque in Cairo and had reportedly been prevented from moving freely by security forces.
The countrywide violence has so far left seven people dead.
In response, the government has promised to crack down on demonstrations and arrest those participating in them. It has blocked internet, mobile phone and SMS services in order to disrupt the planned demonstrations.
Internet shut down
Before Egypt shut down internet access on Thursday night, activists were posting and exchanging messages using social networking services such as Facebook and Twitter, listing more than 30 mosques and churches where protesters were to organise on Friday.
"Egypt's Muslims and Christians will go out to fight against corruption, unemployment and oppression and absence of freedom," a page with more than 70,000 signatories said.
The Associated Press news agency reported that an elite special counterterrorism force had been deployed at strategic points around Cairo, and Egypt's interior ministry warned of "decisive measures".
Safwat Sherif, the secretary-general of the ruling National Democratic Party, told reporters on Thursday: "We hope that tomorrow's Friday prayers and its rituals happen in a quiet way that upholds the value of such rituals ... and that no one jeopardises the safety of citizens or subjects them to something they do not want."
Meanwhile, a lawyer for the Muslim Brotherhood said that 20 members of the officially banned group had been detained overnight.
Abdel-Moniem Abdel-Maksoud said two of the most senior movement members were detained: Essam El-Erian, its main spokesman, and Mohammed Moursi, a prominent Brotherhood leader.
On Thursday, protesters hurled petrol bombs at a fire station in Suez, setting it ablaze. They tried but failed to set fire to a local office of the ruling National Democratic Party. At another rally near Giza on the outskirts of Cairo, police used tear gas to break up hundreds of protesters late at night.
Cairo, normally vibrant on a Thursday night ahead of the weekend, was largely deserted, with shops and restaurants shut. In the city of Ismailia, hundreds of protesters clashed with police who used tear gas and batons to disperse them.
"This is a revolution," one 16-year-old protester said in Suez . "Every day we're coming back here."
"The intensity continues to increase," Al Jazeera's Jamal Elshayyal reported from Suez.
"There have been fierce clashes with rubber-coated steel bullets being fired by the riot police as well as tear gas."
Human Rights Watch said Egyptian police had escalated the use of force against largely peaceful demonstrations and called it "wholly unacceptable and disproportionate".
Barack Obama, the US president, urged both the government and protesters to show restraint as they expressed their "pent-up frustrations". Hillary Clinton, the secretary of state, had earlier said that the protests offered the Mubarak government an opportunity to institute social, economic, and political reforms.
"It is very important that people have mechanisms in order to express legitimate grievances," Obam said as he answered questions from an online audience on the YouTube website.
Obama also urged Mubarak to make changes to the political system to appease the angry protesters.
"I've always said to him that making sure that they are moving forward on reform - political reform, economic reform - is absolutely critical for the long-term well-being of Egypt."