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.
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.