Egyptian protesters torched buildings in Cairo and tried to disrupt international shipping on the Suez Canal at the weekend after a court ruling on a football riot stoked anger.
The ruling enraged residents of Port Said by confirming the death sentences imposed on 21 local football fans for their role in the riot last year in which 74 people were killed.
But the court also angered rival fans in Cairo by acquitting a further 28 defendants, including seven members of the police force.
Two people died in Cairo from the effects of tear gas and rubber bullets and a total of 65 people were injured.
Islamist groups and parties backing President Mohamed Morsi warned against a looming security breakdown and called on their followers to form popular protection committees to guard the streets and public property should police fail to do so.
The stadium riot took place last year at the end of a match in Port Said between the local side al-Masry and Cairo's al-Ahly team.
Spectators were crushed when panicked crowds tried to escape from the stadium after a pitch invasion by al-Masry supporters. Others fell or were thrown from terraces.
Judge Sobhy Abdel Maguid said the Cairo court had confirmed "the death penalty by hanging."
He also sentenced five more people to life imprisonment while others out of a total of 73 defendants received shorter terms.
In Cairo, local al-Ahly fans vented their rage at the acquittals, setting fire to a police social club, the nearby offices of the Egyptian football federation and a branch of a fast food chain.
In Port Said, where the army had taken over security from the police on Friday, about 2,000 residents who want the local fans spared blockaded ferries crossing the Suez Canal and cut adrift speedboats used to supply shipping.
Authorities controlling the canal said through traffic had not been affected.
"The canal is safe and open to all ships passing through it," claimed canal authority spokesman Tarek Hassanein.
Islamist groups the Salafi al-Nour Party and Gama'a al-Islamiyya called on their followers to replace the police force should it pull out, raising fears of further sectarian violence in the streets.
If you appreciated this article then please consider donating to the Morning Star's Fighting Fund to ensure we can keep developing your paper.
Foreign Minister Alistair Burt's admission that the Cameron government has "supported" a survey of attitudes to US drone strikes in Pakistan's tribal areas amounts to a tacit admission of British involvement.
As Britain faces a new housing crisis we can learn from an occasion when tenants banded together to beat their landlord - and won new council housing
Iain Duncan Smith's brainchild came into force at the end of last month. It's bad news for almost everyone