The city at the northern end of the Suez Canal has been a flashpoint since January, with violent protests over death sentences given to residents in connection with a football stadium riot in which more than 70 people died last year.
Security has deteriorated in Egypt since the overthrow of President Hosni Mubarak in a popular uprising two years ago, with some of the worst unrest in Port Said, an industrial city where residents complain of being marginalised.
Youths threw stones and made lewd gestures at a line of police officers who released smoke bombs into streets strewn with rubbish, television footage from pan-Arab channel al-Arabiya showed.
At least six people have been killed in this week's protests in the Mediterranean city, including three policemen.
The security services in the city told Reuters they were ramping up protection of the central prison and Suez Canal offices before a court hearing on Saturday, which is expected to confirm the death sentence for 21 prisoners.
On Thursday the clashes prevented some of the staff of a Suez Canal container company from going to work.
Underlining the overwhelming task facing authorities trying to restore order, police strikes spread across Egypt, with officers demanding better protection.
Hundreds of police went on strike for a second day at a base outside Ismailia, about 70km (45 miles) south of Port Said, demanding more arms after several of their colleagues were killed in recent clashes.
Police also went on strike in Tanta, north of Cairo, while security forces charged with guarding one of President Mohamed Mursi's residences in the Nile Delta started a sit-in at a sports stadium to demand legal protection from prosecution over their actions to control civil unrest.
They said they did not want to get mixed up in the country's political crisis, state news agency MENA reported.
Dozens of police protested peacefully in Egypt's second city of Alexandria, saying that they did not want their work to become politicized and chanting "the police are not against the people", MENA said.
