CAIRO — Egypt’s new government lost control of a major city, Port Said, on Saturday as rampaging soccer fans attacked the main jail, drove police officers from the streets and cut off all access to the city.

Set off by the sentencing of 21 Port Said soccer fans to death, the rioting was the sharpest challenge yet to the efforts of Egypt’s new Islamist rulers to re-establish order after the two years of turmoil that have followed the overthrow of Hosni Mubarak, Egypt’s autocratic president.