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 the fallout last month over a constitution rushed to approval by President Mohammed Morsi, has contributed to a deep national rift over the legitimacy and effectiveness of the country's first democratically elected leader.
But the tensions that have rippled from Friday into Saturday, sweeping up 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 former president 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 in the coastal city in February last year. 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 footage showed protesters fighting police amid clouds of tear gas outside the prison complex and along side streets on 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.
In the two years since a popular uprising ousted Mubarak, Egyptians spanning the spectrum from rights groups and victims of violence and corruption to defendants and their supporters have complained of a highly politicized and ineffective justice system. Many say the courts have failed to adequately investigate charges and that verdicts are often issued less as the culmination of a comprehensive trial than as token gestures to appease popular anger.
Residents of Port Said on Saturday described the verdict as political - a misplaced punishment designed to placate al-Ahly's die-hard fans, known as Ultras, who claim most of the victims as their own. As police fired tear gas and rubber bullets at protesters outside the prison, residents also removed the Egyptian flag from several city buildings in a show of opposition to a government they felt had betrayed them, several news outlets reported. Other men tried to storm at least two police stations, local media reported.
"It was a political ruling and nothing more," said Mohamed Zakaria, a representative of the Port Said teacher's union. "They want to minimize the protests in Tahrir and get the Ultras on the side of the ruling authorities," he said.
In anticipation of an unsatisfactory verdict in the Port Said case earlier this week, the al-Ahly Ultras took to the streets of Cairo, temporarily occupying subway tracks and leaving threatening graffiti on the city's walls. The Port Said violence last year was the deadliest soccer riot in the world in 15 years.
Tens of thousands of opposition protesters also took to the streets Friday to voice opposition to Morsi. Clashes erupted between the protesters and police on the fringes of Cairo's Tahrir Square and outside the presidential palace and state TV building. Protesters also attacked offices belonging to the Muslim Brotherhood, which backs Morsi.
The National Salvation Front, a loose and often divided alliance of opposition parties and activists, said Saturday that it would boycott Egypt's upcoming parliamentary elections if its demands that Morsi form a "national salvation" government and abandon the country's newly ratified constitution were not met.
Morsi is Egypt's first democratically elected president. The constitution, which critics charge has opened the door to a stricter interpretation of Islamic law, was overwhelmingly approved in a national referendum last month.