(By Reuters) - Armored vehicles and military police were deployed on the streets of the Mediterranean city. The state news agency quoted a general as saying the military was sent to "establish calm and stability in Port Said and to protect public institutions".
Security sources said the latest deaths brought to 20 the number killed in three days of violence, and hundreds have been injured.
The unrest began with rallies to mark the second anniversary of the overthrow of autocrat Hosni Mubarak in a revolution which the protesters accuse current President Mohamed Mursi and his Islamist allies of betraying.
The schism is hindering efforts by Mursi, elected in June, to revive an economy in crisis and reverse a plunge in Egypt's currency by enticing back investors and tourists.
Nine people were killed in Friday's violence, most in the port city of Suez, where the army has also been deployed.
Saturday's violence in Port Said erupted when a court sentenced 21 men, most of them from the city, to death for involvement in the disaster in the city's soccer stadium on February 1, 2012.
Many spectators were crushed and witnesses saw some thrown off balconies after the match between Cairo's Al Ahly and local team al-Masri. Many of those killed were from the visiting team's supporters.
Families of victims in court cheered and wept for joy when Judge Sobhy Abdel Maguid read a list of 21 names "referred to the Mufti", a phrase used to denote execution, as all death sentences must be reviewed by Egypt's top religious authority.
A total of 73 people have been standing trial. Other rulings will be issued on March 9, the judge said.
One relative of a victim in the court shouted: "God is greatest." Outside Al Ahly club in Cairo, supporters also cheered. Fans had threatened fresh violence unless the death penalty was meted out.
GUNSHOTS NEAR PRISON
But in Port Said residents rampaged through the streets in anger that people from their city had been blamed. Gunshots were reported near the prison where most of the convicted men are being held.
One security source reported 11 killed in the violence, while two other sources put Saturday's toll at 12. At least two of the dead were policemen.
A witness said some men stormed a police station.
Thousands took to the streets of Cairo, Alexandria and other cities on Friday to protest against what they call the authoritarianism of Mursi's rule.
"We want to change the president and the government. We are tired of this regime. Nothing has changed," said Mahmoud Suleiman, 22, in Cairo's Tahrir Square, the cauldron of the 2011 revolt. Nearby, youths hurled stones at police early on Saturday.
"The protests will continue until we realize all the demands of the revolution - bread, freedom and social justice," said Ahmed Salama, 28, a protester camped out with dozens of others in Tahrir.
Mursi's supporters say their critics are ignoring democratic principles, after elections swept the Islamists to office.
In a statement in response to Friday's violence, Mursi said the state would not hesitate in "pursuing the criminals and delivering them to justice". He urged Egyptians to respect the principles of the revolution by expressing views peacefully.
The president was due to meet later on Saturday with the National Defence Council, which includes senior ministers and security officials, to discuss the violence.
Unrest has been stoked by Mursi's decision to fast-track an Islamist-tinged constitution rejected by his opponents.
Inspired by the popular uprising in Tunisia, Egypt's revolution spurred further revolts across the Arab world. But the sense of common purpose that united Egyptians two years ago has given way to internal strife that triggered bloody street battles last month.
(Writing by Edmund Blair; Editing by Andrew Roche)