Clashes killed at least 31 people in Egypt's Port Said as violence raged into the early hours of Sunday in several cities including the capital following death sentences passed on 21 football fans after a riot.

The unrest came after a day of deadly protests against Islamist President Mohamed Morsi, highlighting deep political divisions in the country and long-standing tensions between police and protesters.

Trouble flared just minutes after a court on Saturday handed down the death sentences against fans of Port Said club Al-Masry after 74 people were killed in post-match violence last February following a match with Cairo side Al-Ahly.

Health ministry spokesman Ahmed Omar said 31 people died in the canal city.

In Cairo, police clashed with protesters on the outskirts of Tahrir Square - the symbolic heart of the revolt that ousted Hosni Mubarak in 2011 - firing volleys of tear gas, witnesses said.

Demonstrators blocked the October 6 bridge, a vital flyover linking east and west Cairo, as police and masked protesters clashed on the Nile corniche.

Many Egyptians believe last year's deadly stadium riots in Port Said were orchestrated.AFP