second anniversary of Egypt's uprising against predecessor Hosni Mubarak, the worst crisis he has faced since taking power in June.
Trouble erupted just minutes after a Cairo court handed down the sentences against fans of Port Said side Al-Masry over the deaths of 74 people during post-match violence last February.
Protesters attacked police stations and set tyres alight and relatives of those who were sentenced clashed with security forces as they tried to storm the prison in Port Said where the defendants are being held.
Some of the attackers used automatic weapons against police who responded with tear gas, witnesses said.
The clashes left at least 30 people dead and 312 wounded, the health ministry said. The interior ministry said two policemen were among those killed. Medics told AFP all the fatalities were from gunfire.
Crowds stormed two police stations as heavy shooting crackled through the city, where shops and businesses had closed, an AFP correspondent said.
Ambulances ferried the injured to hospitals and mosques urged worshippers to donate blood.
The army deployed troops to restore calm and protect vital public buildings, military sources and witnesses said.
Clashes also erupted in the nearby canal city of Suez, where at least eight people were killed in fighting on Friday. Protesters attacked a police station, freeing 25 detainees and seizing weapons, security sources said.
The opposition, meanwhile, threatened to boycott upcoming parliamentary polls if Morsi did not find a "comprehensive solution" to the unrest.
The National Salvation Front, the main coalition of parties and movements opposing the ruling Islamists said it would "not participate" in the polls unless a "national salvation" government was formed.
Egypt's national defence council, which is headed by Morsi, appealed for calm and called for a dialogue with "independent national figures" to agree on a mechanism for the polls.
European Union foreign policy chief Catherine Ashton on Saturday expressed "great concern" over the violence, urging the authorities "to restore calm and order" and appealing for restraint on all sides.
'I am satisfied'
Inside and outside the court on Saturday there were explosions of joy at the verdict. Women ululated, relatives hugged each other and shouted "Allahu akbar" (God is greatest).
"I am satisfied with the verdict," said a man whose son was killed in the Port Said violence.
Hassan Mustafa, who wore a picture of his dead friend pinned to his chest, said he still wanted "justice served for those who planned the killing."
Many Egyptians believe the violence, during which fans of Al-Masry attacked players and fans of Cairo's Al-Ahly, was orchestrated either by police or by supporters of Mubarak.
Egypt's top cleric must ratify Saturday's verdicts, as is customary, and the sentences are subject to appeal.
Verdicts will be announced on March 9 for another 52 defendants, including police officers.
The army sent reinforcements to protect the strategic Suez Canal, which connects the Mediterranean and the Red Sea and is one of the world's busiest shipping lanes, a military source told the state-run MENA news agency.
Troops also deployed in Port Said to protect vital installations including a power station, a water station, banks and administrative buildings linked to the canal, which generates some five billion dollars in annual revenues.
A Greek ferry anchored in Port Said was hit by apparent stray gunfire on Saturday, Greece's foreign ministry said. No one was hurt.
Tens of thousands took to the nation's streets on Friday to protest against Morsi, who they accuse of failing the revolution and consolidating power in the hands of his Muslim Brotherhood.