• Soldiers told to restore order as violent clashes continue • Mohamed ElBaradei placed under house arrest • Many police switching sides and joining protests

President Hosni Mubarak has ordered tanks and troops onto the streets of Egypt to enforce a curfew as violent clashes continue between police and protesters demanding an end to his 30-year rule.

Towards the end of a day that saw the Nobel peace laureate Mohamed ElBaradei placed under house arrest and the headquarters of the ruling party set ablaze, army tanks rolled into the centre of Cairo and Suez, where they were greeted by demonstrators hoping that the army would side with them against the police.

In Cairo's main square thousands of anti-government protesters wielding rocks, glass and sticks chased hundreds of riot police away. Several police officers stripped off their uniforms and badges and joined demonstrators.

The scenes were repeated in the cities of Alexandria, Assiut, Minya, al-Arish, Suez and Mansoura. There were protests in 11 out of the 28 Egyptian provinces.

The curfew, which will cover Cairo, Alexandria and Suez, began at 6pm Egypt time and will run until 7am. But with thousands of protesters still on the streets it is hard to see how Mubarak's emergency measure will be enforced.

Earlier today ElBaradei told the Guardian that the Mubarak government was about to fall, saying: "Egypt is being isolated by a regime on its last legs."

The former UN weapons inspector's rhetoric seems to have drawn the ire of the government. Before being placed under house arrest at his Cairo home this afternoon ElBaradei was attacked by police firing water cannon as he and supporters joined the protest.

Officers beat his supporters with batons and a soaking ElBaradei was trapped inside a Giza mosque while hundreds of riot police laid siege to it, firing teargas in the streets around so no one could leave.

Elbaradei's detention was criticised by the International Crisis Group, which said the act had no credible basis.

"In a situation as tense as this, repression and abuse can only further inflame the situation. Rather than resort to repression the authorities should heed demands of the population for dramatic political, social and economic transformation."

The demonstrations were energised by both the return of ElBaradei and the backing of the country's biggest opposition group, the Islamic fundamentalist Muslim Brotherhood, which urged its supporters to join the protests.

Several police officers appear to have heeded the protesters' pleas to join them. A policeman in Cairo was seen discarding a teargas canister to show demonstrators that he was on their side.

The chaotic and violent scenes mark a serious escalation in tensions between the protesters and the Mubarak regime.

Security officials said protesters ransacked the headquarters of Mubarak's ruling party in the cities of Mansoura, north of Cairo, and Suez, east of the capital.

Some of the most serious violence was in Suez, where protesters seized weapons stored in a police station and asked the policemen inside to leave the building, then burned it down. They set ablaze about 20 police trucks parked nearby. At least one protester was shot dead – the eighth in the last four days.

Internet and mobile phone services, at least in Cairo, appeared to be largely cut off since last night in the most extreme measure so far to try to hamper protesters. It did not stop tens of thousands flooding the streets, emboldened by the recent uprising in Tunisia.

In Cairo protesters complaining of police violence called on the army to join them, chanting: "Where is the army? Come and see what the police is doing to us. We want the army. We want the army."

In the affluent Mohandiseen neighbourhood at least 10,000 were marching toward the city centre chanting "Down, down with Mubarak." The crowd swelled to about 20,000 as they made their way through residential areas.

At Ramsis square in the heart of the city thousands clashed with police as they left the al-Nur mosque after prayers. Police used teargas and rubber bullets. Some of the teargas was fired inside the mosque where women were taking refuge. Hundreds of people later broke through police cordons to head to the main downtown square, Tahrir. They were stopped by police firing teargas. Similar scenes played out in Alexandria.

Peter Bouckaert, emergencies director of Human Rights Watch, said about 1,000 peaceful protesters were attacked by police in armoured cars as they emerged from a mosque.

"Fierce clashes started then, with exchanges of rock throwing," he said. "About 200 police faced about 1,000 protesters. The clashes lasted for nearly two hours. Then a much larger crowd of protesters came from another direction. They were packed in four blocks deep. Police tried to hold them back with teargas and rubber bullets but they were finally overwhelmed."

"Then the police just gave up, at about the time of afternoon prayers. Protesters gave water to police and talked to them. It was was all peaceful.

"Now walking down to downtown Alexandria, the whole road is packed as far as we can see, people shouting slogans against [Hosni] Mubarak and his son Gamal. It is a very festive atmosphere. Women in veils, old men, children, I even saw a blind man being led. And there are no police anywhere."