Egyptian President Mohammed Morsi has declared a state of emergency in the cities of Port Said, Suez and Ismalia after days of deadly unrest.
A curfew from 21:00 to 06:00 will be imposed for 30 days from Monday.
At least 33 people died at the weekend in Port Said, where a court judgment sparked rioting. Unhappiness with Mr Morsi's rule fuelled unrest elsewhere.
Mr Morsi has invited opposition politicians to a "national dialogue" meeting on Monday.
'My duty'
Violence continued overnight, with anti-Morsi protesters in Ismalia clashing with police, who responded with tear gas.
On Sunday in the capital, Cairo, anti-government protesters clashed with security forces near Tahrir Square for a fourth consecutive day.
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Amr MubarakPort Said residentWe will not allow our children to be scapegoats for Morsi”
- Analysis: Violence tests president
The liberal opposition accuses Mr Morsi of being autocratic and driving through a new constitution that does not protect adequately freedom of expression or religion.
The constitution was approved in a national referendum in December.
The government is also being blamed for a deepening economic crisis.
The state of emergency applies to the three cities along the Suez Canal, and their surrounding regions.
In a national televised address, Mr Morsi said he might take further steps "for the sake of Egypt" as it was his "duty" as president.
"I have said I am against any emergency measures but I have said that if I must stop bloodshed and protect the people, then I will act," the president said.
"If I must, I will do much more for the sake of Egypt. This is my duty and I will not hesitate."
He also invited political leaders to the national dialogue meeting at 18:00 local time (16:00 GMT) on Monday.
The BBC's Yolande Knell, in Cairo, says Mr Morsi is hoping that political dialogue can restore national unity but it is not clear who will accept his invitation.
Egypt's main opposition coalition, the National Salvation Front, said it would meet on Monday morning to discuss the initiative.
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Port Said 2012 football deaths
- 74 people killed in Port Said stadium on 2 February 2012
- Clashes broke out between rival fans of clubs al-Masry and al-Ahly
- Fans flooded on to pitch attacking Ahly players and fans as match ended
- Most died of concussion, cuts and suffocation
- The largest death toll in Egypt's football history
Front spokesman Khaled Dawoud told Reuters news agency: "[Mr Morsi's] call to implement emergency law was a right move given what is going on, namely thuggery and criminal actions."
However, he said that Mr Morsi was "missing the real problem on the ground, which is his own polices".
Former presidential candidate Hamdeen Sabahy said he would not attend Monday's meeting "unless the bloodshed stops and the people's demands are met".
On his Twitter account, leading Front politician Mohamed ElBaradei called the dialogue "a waste of time".
Protests erupted in Port Said after a court sentenced 21 local people to death over riots that killed 74 people after a football game last February.
The violence began when fans of Port Said side al-Masry attacked visiting supporters from Cairo club al-Ahly.
Fans flooded on to the pitch, attacking Ahly players and fans as the match ended.
Most of the victims died of concussion, cuts and suffocation.
About 30 people were killed in unrest in Port Said on Saturday, with a further three dying at a mass funeral on Sunday.
