Egyptian military forces also were sent into the canal city of Suez after eight people died in Friday's clashes between security forces and protesters opposed to the new president and the Brotherhood. Another protester was killed in Ismailiya, and security officials told the state news agency MENA that two policemen were killed in Friday's protests.
Many of the young men who led the protests and clashes hail from the Ultras. They often come from poor neighborhoods and view the police force that was the backbone of Mubarak's authoritarian rule as their nemesis.
"The police are thugs!" yelled relatives of the deceased inside the courtroom before the judge took the bench.
Near Cairo's Tahrir Square, where tens of thousands had amassed to mark the two-year anniversary a day earlier, Ultras Al-Ahly waved their team's red flag as they clashed with police who fired tear gas to disburse the crowd near Cabinet headquarters and Parliament.
Underlining the distrust that lingers between much of the public and the police, survivors and witnesses say Mubarak loyalists had a hand in instigating last year's attack, which began after Port Said's home team won the match, 3-1, and that the police at the very least were responsible for gross negligence.
Al-Masry fans stormed the pitch after the game ended, attacking Cairo's Al-Ahly fans. Authorities shut off the stadium lights, plunging it into darkness. In the exit corridor, the fleeing crowd pressed against a chained gate until it broke open. Many were crushed under the crowd of people trying to flee.
Other survivors said it was simply bloodthirsty Al-Masry fans and lack of enough security that led to the deaths of their colleagues, although there has long been bad blood between the two rival teams, both sides blame police for failing to perform usual searches for weapons at the stadium.
Anger is boiling in Port Said, where residents say they have been unfairly scapegoated.
A lawyer of one of the defendants given a death sentence said the verdict was political.
"There is nothing to say these people did anything and we don't understand what this verdict is based on," Mohammed al-Daw told The Associated Press by telephone.
"Our situation in Port Said is very grave because kids were taken from their homes for wearing green T-shirts," he said, referring to the Al-Masry team color.
Nine security officials who also were on trial Saturday were acquitted, lawyers and security officials say.
Al-Daw and other defense attorneys said all those sentenced were Al-Masry fans. As is customary in Egypt, the death sentences will be sent to the nation's top religious authority, the Grand Mufti, for approval, though the court has final say on the matter.
Fans of Al-Ahly, whose stands were attacked by rival club Al-Masry in the incident in Port Said, had promised more violence in the days leading up to the verdict if the death penalty was not handed down.
Before the judge could read out the names of the 21, families erupted in relief, yelling "Allahu Akbar!" Arabic for "God is great," with their hands in the air and waving pictures of the deceased. One man fainted while others hugged one another. The judge smacked the bench several times to try to restore calm in the courtroom.
"This was necessary," said Nour al-Sabah, whose 17-year-old son Ahmed Zakaria died in last year's melee. "Now I want to see the guys when they are executed with my own eyes, just as they saw the murder of my son."
Thousands of Al-Ahly fans gathered outside the Cairo sports club for the verdict, chanting against the police and the government.
"We are not really that happy," Mohamed Ahmed, a survivor of the attack, said. "The government helped the Ultras of Port Said by blocking the gates of the stadium until people suffocated to death.
Associated Press writer Mariam Rizk contributed to this report.
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