However, canal authorities said shipping passing along the canal had been unaffected by the protests in the city, sited at the waterway's northern end, state news agency MENA reported.
In Cairo, fans of a rival soccer team set fire to a police social club and the nearby offices of the Egyptian Soccer Federation, sending smoke rising over the capital. They were enraged by the same court clearing 28 other defendants of blame over the riot, in which more than 70 people were killed.
The stadium deaths took place last year at the end of a match between Cairo's Al-Ahly team and Port Said's Al-Masry. Spectators were crushed when panicked crowds tried to escape from the stadium after a pitch invasion by supporters of Al-Masry. Others fell or were thrown from terraces.
Unrest has plagued Port Said since the death sentences were first handed down to the Al-Masry supporters in January, with locals who want the fans spared fighting pitched battles with police. At least eight people have been killed this week, including three policemen.
The Cairo court also jailed two senior police officers for 15 years on Saturday for their handling of the riot.
Witnesses said youths in Port Said untied moored speedboats used to supply shipping on the waterway, hoping the boats would drift into the path of passing vessels. About 2,000 people also blocked car ferries from crossing the canal.
Military police recovered five speedboats and brought them back to shore, but two were still drifting, one witness said. The army has been in charge of security in central Port Said after police were pulled out on Friday to ease tensions.
The canal is a major employer in the city and, until now, protesters had declared it off-limits for the demonstrations apart from on one occasion when red balloons marked "SOS" were floated into the waterway.
Some fans of the Cairo side, angry about the acquittals, torched the buildings in the capital on the banks of the Nile. However, others were happy with the confirmation of the death sentences. "This is a just verdict and has calmed us all down. Our martyrs have been vindicated," Said Sayyid, 21, told Reuters.
The riot case has highlighted deteriorating law and order since the 2011 revolution as public anger explodes overIslamist President Mohamed Mursi's failure to reform a police force hated for its brutality under deposed leaderHosni Mubarak.
Judge Sobhy Abdel Maguid, listing the names of the 21 fans, said the Cairo court had confirmed "the death penalty by hanging". He also sentenced five more people to life imprisonment while others out of a total of 73 defendants received shorter terms.
Writing by David Stamp; Editing by Pravin Char