Angry youths hurled rocks at security forces and burned a police truck as thousands gathered in central Cairo to protest against Egyptian President Mohammed Morsi's decision to grab sweeping new powers.
Police fired tear gas near Tahrir Square, heart of the 2011 uprising that toppled Hosni Mubarak at the height of the Arab Spring. Thousands demanded that Mr. Morsi should quit and accused him of launching a "coup."
There were also violent protests in Alexandria, Port Said and Suez.
Mr. Morsi on Thursday issued a decree that puts his decisions beyond any legal challenge until a new parliament is elected. Opponents immediately accused him of turning into a new Mubarak and hijacking the Egyptian revolution.
The United States, the European Union and the United Nations expressed concern at Mr. Morsi's move.
Mr. Morsi's rivals condemned him as an autocratic pharaoh who wanted to impose his Islamist vision on Egypt.
Egyptian judges will meet on Saturday to respond to Mr. Morsi's move, which puts him above the judicial oversight. The judges could threaten to go on strike, which would bring the judiciary to a halt.
Mr. Morsi's decree has consolidated his power but looks set to polarize Egypt further, threatening more turmoil in a nation at the heart of the Arab Spring.
In Alexandria, north of Cairo, protesters ransacked an office of the Muslim Brotherhood's political party, burning books and chairs in the street. Supporters of Mr. Morsi and opponents clashed elsewhere in the city, leaving 12 injured.
A party building was attacked by stone-throwing protesters in Port Said, and demonstrators in Suez threw petrol bombs that burned banners outside the party building.
Although Washington has praised Egypt for its part in bringing Israelis and Palestinians to a ceasefire on Wednesday, it expressed reservations about Mr. Morsi's latest move.
"The decisions and declarations announced on Nov. 22 raise concerns for many Egyptians and for the international community," State Department spokeswoman Victoria Nuland said in a statement.
The European Union urged Mr. Morsi to respect the democratic process, while the United Nations expressed fears about human rights.
"The decree is basically a coup on state institutions and the rule of law that is likely to undermine the revolution and the transition to democracy," said Mervat Ahmed, an independent activist in Tahrir protesting against the decree.
Leading liberal Mohamed ElBaradei, who joined other politicians on Thursday night to demand the decree be withdrawn, wrote on his Twitter account that Mr. Morsi had "usurped all state powers and appointed himself Egypt's new Pharaoh."
Almost two years after Mr. Mubarak was toppled and about five months since Mr. Morsi took office, Egypt has no permanent constitution, which must be in place before new parliamentary elections are held.
An assembly drawing up the constitution has yet to complete its work. Many liberals, Christians and others have walked out accusing the Islamists who dominate it of ignoring their voices over the extent that Islam should be enshrined in the new state.