Chaos broke out as soon as the verdict was pronounced as family members and friends of those sentenced clashed with security forces, attacked police stations and tried to storm the prison holding the convicts.
The violence came at a time when unrest is sweeping Egypt on the second anniversary of the uprising that ousted Hosni Mubarak. Clashes marking the revolution's second anniversary on Jan 25 left at least nine people dead and 530 injured.
Two police officers and 28 civilians were killed and over 300 injured in the clashes since the verdict was read this morning, said a report in Al Ahram newspaper. "There are many more severely wounded... which means that the death toll can rise within the coming hours," said Port Said Hospital directorate, Abdel Rahman Farah.
Security forces fired tear gas to disperse the angry crowd trying to break into the prison, as family members of those sentenced screamed and wailed across the streets of the city.
All 21 defendants sentenced to death were fans of Port Said club al-Masry, who were accused of attacking and killing fans of the Cairo club al-Ahly during a match. While supporters of the al-Masry fans went on a rampage, the verdict was greeted by cheers from relatives of the victims and al-Ahly fans.
A section of al-Ahly supporters, known as the "ultras", had played a prominent role in the protests against Mubarak, and many accused supporters of the former president of instigating the violence in the Port Said match last year.
Taken aback by the level of violence, security officials said the military is being deployed to Port Said. This is the second such deployment in less than 24 hours. The army has already taken control of the Suez Canal out of fear that the violence might affect the Canal.
Army was forced to deploy its forces in the Suez after the security affiliated with the Interior Ministry lost control there last night after eight died in clashes with security there.
As news of the verdict spread, the families of the sentenced gathered in front of a prison in Port Said. Reports said unidentified assailants used automatic weapons against police. Two policemen were shot dead outside the jail.
Two policemen were among those killed, the Interior Ministry said in a statement, adding that there were "many critical injuries among police forces."
A senior Army officer said that troops are being sent to Port Said. "It has been decided to deploy some units to work for calm and stability and the protection of public establishments," General Ahmed Wasfi said in a statement carried by the official MENA news agency.
According to witnesses, shops have closed down and armoured personnel vehicles have been deployed. The verdict came as surprise for many as all experts expected acquittal.
The judge said in his statement read live on state TV that he would announce the verdict for the remaining 52 defendants on Mar 9. Among those on trial are nine security officials.
The soccer melee on Feb 1, 2012 between Port Said's Al-Masry fans and Cairo's Al-Ahly fans was the world's deadliest soccer violence in 15 years.
As is customary in Egypt, the death sentences will be sent to a top religious authority, the Grand Mufti, for approval. Much has elapsed since the mass uprising of 2011 overthrew Mubarak, and Egyptians have for the first time elected a government of their own.
But the developments have left the country sharply polarised. The presidential vote that elected Brotherhood leader Mohamed Mursi as the president was far from overwhelming for any single candidate.
A recent referendum on the constitution also left the country deeply divided with the secular and liberal opposition terming the draft as "too Islamist" that would compromise on the rights of the minorities.
Mursi took to Twitter to appeal for calm, urging "citizens to adhere to the values of the revolution, express opinions freely and peacefully and renounce violence".