Protests have erupted across Egypt and the offices of ruling Muslim Brotherhood party have been attacked.
Protests in Egypt have entered a second day as activists announced a week-long sit-in in Cairo's Tahrir Square.
The demonstrations, which were sparked by Egyptian president Mohamed Morsi's decision to grant himself controversial new powers that would put him above the judiciary, started up again on Saturday.
Insisting upon the need to root out what he called "weevils eating away at the nation of Egypt", Mohamed Morsi said on Friday: "I don't like, want or need to resort to exceptional measures, but I will if I see that my people, nation and the revolution of Egypt are in danger".
Morsi - who has been buoyed by accolades recently for mediating a truce between Hamas and Israel - had ordered on Thursday that an assembly dominated by the Muslim Brotherhood writing the new constitution could not be dissolved by legal challenges.
Liberal and secular members earlier walked out of the body, charging it would impose strict Islamic practises.
His announcement led to clashes in several cities between supporters and opponents of Egypt's president, a clear show of the deepening polarisation plaguing the country.
The Obama administration has voiced concern about developments.
Both supporters and critics of the president demonstrated across the country on Friday.
Overnight, some 20 white tents were pitched in the middle of Cairo's totemic Tahrir Square, where Egyptians celebrated the ousting of President Hosni Mubarak last year.
Protest organisers said more than 20 different groups had joined the week-long sit-in against Mr Mursi's reforms.
They say the new president is becoming as much of a dictator as long-time ruler Hosni Mubarak, who was overthrown by the 2011 uprising.
Numbers of protesters in the square have ranged from a few hundred to a few thousand.
The protesters have two demands: to reverse the president's constitutional declaration, and to dissolve the constituent assembly that is currently drafting Egypt's new constitution.
But by Saturday morning there were few protesters left.
Dozens of messages on Twitter said protesters were forced to abandon the camp after being targeted with tear gas.
Separately on Friday evening another set of anti-Mursi protesters clashed with police near the interior ministry off Tahrir Square. Police fired tear gas and demonstrators threw petrol bombs.
Chants of "Mursi is Mubarak... revolution everywhere" rang out.
More than 100 people were injured in violence as offices of the president's Muslim Brotherhood party were reportedly attacked in the cities of Port Said, Suez and Ismailia, while clashes between rival demonstrations took place in Alexandria.
Defending his decision at a rally at Cairo's presidential palace on Friday, Mursi said he was the guardian of political, economic and social stability, but that he wanted to see a "genuine opposition, a strong opposition".
Key opposition figures have accused the president of "monopolising all three branches of government" and overseeing "the total execution of the independence of the judiciary".
US state department spokeswoman Victoria Nuland said on Friday that the decree had "raised concerns" in the international community.
She said Egypt's revolution had aimed "to ensure that power would not be overly concentrated in the hands of any one person or institution".
The US wanted democratic dialogue within Egypt to solve constitutional issues, said Nuland.
In recent days, Mursi had been praised by the US and other nations for his efforts in brokering the ceasefire in Gaza.
The new decree bans challenges to Mr Mursi's laws and decisions, and says no court can dissolve the constituent assembly, which is drawing up a new constitution.
It also opens the way for a retrial of people convicted of killings during Egypt's 2011 uprising which toppled Mubarak.
The ousted leader is serving a long jail term for ordering the killing of protesters in 2011, but others in his administration were acquitted.
The declaration also gives the 100-member constituent assembly two additional months to draft a new constitution, to replace the one suspended after Mubarak was overthrown.
The rewrite of the constitution, which was meant to be finished by December, has been plagued by legal complaints questioning the make-up of the constituent assembly.
Once completed, the document is due to be put to a referendum. If it is approved, legislative elections will be held two months later.