The Egyptian military deployed to Suez and Port Said on Saturday, and the Interior Ministry said Saturday night that it had arrested 130 people, across nine of Egypt's 27 provinces, in connection with the violence.
Information Minister Salah Abdel-Maksoud also said the National Defense Council would consider implementing a curfew and a state of emergency as violence persisted in Cairo, Port Said and Suez.
All of those killed in Port Said on Saturday died of gunshot wounds, as protesters angered by the court verdict battled police outside the prison where the defendants were being held and at police stations, said Abdel Rahman Farah, the manager of the city's hospitals. Two soccer players, including a former member of Port Said's Al-Masry club, were among the dead, the Associated Press reported.
At least 300 people were injured in the city, Farah said.
Anger toward Egypt's new Islamist government, as well as fallout last month over a constitution rushed to approval by President Mohamed Morsi, has contributed to a deep national rift over the legitimacy and effectiveness of Morsi, the country's first democratically elected president.
But the tensions that rippled from Friday into Saturday, incorporating a host of popular grievances, also underscore the sense of injustice that lingers in this nation of 85 million two years after the fall of autocratic leader Hosni Mubarak.
In Saturday's verdict, a Cairo judge sentenced to death 21 people charged in the killing of 74 soccer fans in a riot that broke out between fans of the club team Al-Ahly and Port Said's Al-Masry club team after a match last February in the coastal city. A verdict is expected for the remaining 52 defendants in March.
The announcement sparked exclamations of surprise and tears from victims' families in the courtroom and outside the Al-Ahly soccer club in Cairo where fans had gathered. But it sparked anger in Port Said, home to most of the defendants, where their families and supporters tried to storm the prison.
Television news showed protesters fighting police amid clouds of tear gas outside the prison complex and along side streets Saturday afternoon.
"All the shops are closed, and the city is under complete paralysis," said Mohamed, a hotel owner in the center of the city who declined to give his last name out of anxiety about the security situation.
Others reported that residents were stocking up on groceries in preparation for days of clashes.