* 21 face execution over stadium disaster that killed 74
* Death toll in three days of violence hits 25
* Army deployed in Mediterranean city of Port Said
* Cairo soccer fans cheer court ruling (Updates death toll, adds analyst comment)
By Yusri Mohamed and Yasmine Saleh
PORT SAID/CAIRO, Jan 26 (Reuters) - At least 16 people died in a rampage by protesters angry at the sentencing of 21 people to death over a soccer stadium disaster, amid a wave of bloody unrest posing a challenge for Egypt's new Islamist rulers.
Armoured vehicles and military police were deployed on the streets of Port Said after the violence in the Mediterranean city on Saturday. 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".
The latest deaths brought to at least 25 the number reported killed in three days of violence. Hundreds have been injured in clashes in which police have rained down tear gas on protesters armed with stones and some with petrol bombs.
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. The polarisation and lack of security that has blighted Egypt casts a shadow over a parliamentary election expected to start in April.
Mursi's opponents say he has failed to deliver on economic promises or to be a president for all Egyptians, as he pledged. His backers say his critics do not respect the democracy that has given Egypt its first freely elected leader.
Nine people were reported 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 from the city, to death for involvement in the disaster in the city's soccer stadium which killed 74 people on Feb. 1, 2012.
VICTIMS' RELATIVES CHEER
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.
But in Port Said residents rampaged through the streets in anger that men from their city had been blamed. Gunshots were reported near the prison where most are being held.
The director of Port Said hospitals said 16 people were killed and 200 wounded, state television reported. Security sources said at least two of the dead were policemen.
A witness said some men stormed a police station in Port Said, where protesters lit tyres in the street, sending plumes of black smoke in the air.
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.
"NOTHING HAS CHANGED"
"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.
Ahmed Salama, 28, a protester camped out with dozens of others in Tahrir, said: "The protests will continue until we realise all the demands of the revolution - bread, freedom and social justice."
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.
Mustapha Kamal Al-Sayyid, a professor of political science at Cairo University, said violence reflected the frustration of many liberal-minded Egyptians and others.
"The state of polarisation between Islamists and others is most likely to continue and will have very negative impact on the state's politics, security and economy," he said.
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. (Additional reporting by Omar Fahmy; Writing by Edmund Blair; Editing by Andrew Roche)