The fatalities were demonstrators who were shot dead, the sources said.
Thousands had protested in Cairo during the day, accusing the ruling military council, which took power when veteran president Hosni Mubarak was ousted one year ago, of mismanaging the country during a fragile transition.
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State television said 628 people were injured in earlier clashes that erupted in the capital, mostly from tear gas inhalation.
The protests were sparked by Wednesday's deadly violence between fans of Port Said home team Al-Masry and Cairo's Al-Ahly, which marked one of the deadliest incidents in football history.
Those clashes erupted at the final whistle of a match which saw Al-Masry beat Al-Ahly 3-1.
Al-Masry fans invaded the pitch, throwing rocks, bottles and fireworks at Al-Ahly supporters, causing chaos and panic as players and fans fled in all directions, witnesses said.
