CAIRO — Clashes between protesters and security forces left five people dead on Friday in an escalation of violence that threatened to undo Egypt’s halting steps toward stability after the ouster of President Hosni Mubarak one year ago.

The protests began Thursday in anger at the police for failing to prevent a post-match fight between rival groups of soccer fans in Port Said in which 70 people were killed on Wednesday night.

But as the protests continued for a second day, the original riot had become a metaphor for the failure of the military-led government to resolve the complaints that fuel recurring street violence, including brutal but capricious police officers, a lack of accountability and implausible attempts to blame mysterious third parties for social unrest.

If the ruling military council cannot control a soccer game, many asked, how can they run a country of more than 80 million? “If you can’t secure a match, tell me how will you secure Egypt?” demonstrators chanted on their way to the headquarters of the Interior Ministry.