At times in the violence, frictions have arisen between the police that were battling protesters and army forces that tried to break up the fighting. Troops in between the two sides were overwhelmed by police tear gas, one army colonel was wounded by live fire, and troops even opened fire over the heads of police, bringing cheers from protesters.
Three policemen and three civilians were killed in the fighting, and troops stood by as protesters torched a government complex Monday that contains the city's main police building.
The scenes, following three weeks of strikes and protests in the city, have underlined a scenario that many in Egypt view with a mix of concern and relief that the military may move back into politics, prompted by mushrooming protests, a breakdown in law and order and mounting challenges to the Islamist President Mohammed Morsi. Some opponents of Morsi have outright called for the military to take power, and even those who say they oppose a military return have used the prospect to pressure Morsi to find some consensus in the country's political crisis.
Prominent opposition leader Mohammed ElBaradei warned of decaying state institutions and rising levels of violence.
"The regime in its current form is unable to manage the country," he wrote on his Twitter account. "There must be a radical review before it is too late."
Unable to halt the violence, both the police and military Monday sought to deny any tensions between them. Meanwhile, there was no official comment from the presidency following one of the worst flare-ups of violence since January.
Unrest also spread in other parts of the country. In the capital, Cairo, protesters blocked the main thoroughfare along the Nile River, and police tried to clear them with volleys of tear gas. Other disgruntled young men set fire to two police cars in two different locations in Cairo, sending police fleeing the vehicles in the middle of traffic.
The unrest in Egypt is reaching new heights, just weeks before the parliamentary elections scheduled next month, which have further enflamed an already tense political landscape.
Opposition leaders are calling for a boycott of the elections, accusing Morsi of failing to seek consensus over critical issues, such as the drafting of the constitution and the elections law. Morsi opponents accuse him of working to lock his Muslim Brotherhood's hold on power.
"How can elections be held with the tragic situation in Port Said and as martyrs are falling there and in other provinces," former presidential candidate and leading opposition member Amr Moussa said in an email. "How can elections be held with security undermined everywhere in the nation."
Months of violence have prompted calls by some in the opposition for the military, which ruled for nearly 17 months after the fall of autocrat Hosni Mubarak in February 2011, to take back power and contain the discontent.
Hundreds of Port Said residents, followed by some in other provinces, took the symbolic step of going to public registrar offices to issue "power of delegation" documents asking the military to step in "manage" the country.
Abdullah el-Sinawi, a commentator and columnist in the daily Al-Shorouk newspaper, said the "clashes between police and military which took place, even if they were limited, represent the absence of the rule of law."
Such a clash between two powerful institutions "warns of the decay of the state," he said. There were increasing signs from the generals of their displeasure with Morsi's management of the country and lack of political openness, he said, raising the specter of military intervention, perhaps not to remove Morsi but to prevent a breakdown.
Morsi supporters and members of the Muslim Brotherhood have downplayed rising expressions of discontent. Mohammed el-Beltagi, a senior Brotherhood member, told reporters that Egyptians will never accept the military back in power.
Another senior Brotherhood member, Gamal Heshmat, told the legislature Monday that the media is portraying Port Said as if it is "on fire."
"Anyone watching from outside will think that Egypt is all Port Said," he told a parliamentary session, according to the main state-run daily, Al-Ahram.
Port Said, on the Mediterranean coast at the mouth of the Suez Canal, has been in turmoil since late January, when a Cairo court issued death sentences against 21 people, most residents of the city, for involvement in Egypt's deadliest soccer riot in February 2012. The verdicts sparked angry protests in the city, which turned into deadly clashes with police, leaving more than 40 dead, including two policemen.
Residents have been outraged by what they call excessive force by the police and by Morsi's backing of the security forces. The police said most of the protester deaths came during an attempt to storm a prison.
Tensions have risen further ahead of a court hearing planned for Saturday that is expected to confirm the death sentences and issue new verdicts for police officers and other Port Said defendants also charged in the soccer riot case.
Port Said's fighting Sunday began when thousands of protesters marched on the police headquarters after word emerged that 39 defendants in the soccer case had been transferred to prisons outside the city ahead of Saturday's hearing.
The protesters Sunday lobbed rocks and firebombs at the police building. The police, heavily deployed around their building, moved out to try to push back the protesters.
Army officers tried but failed to negotiate an end to the fighting. Then the troops began getting hit. An army colonel was shot in the right leg by live ammunition, military spokesman Ahmed Mohammed Ali said. Soldiers were overwhelmed by the tear gas that police were firing at the protesters. In one case, captured on video, a tear gas canister fell inside a military armored vehicle, and the choking soldiers stumbled out, helped by protesters who carried them to an ambulance.
The military deployed their vehicles between the protesters and police. They fired in the air in the direction of the police, witnesses said. "The people and army are one hand," cheering protesters chanted.
One protester, 25-year-old Mohammed Atef, said the scene restored his confidence in the military. "We felt they are feeling the injustices against us; that they decided to protect us too."
But another said she believed the military was only acting to protect itself. "The military only moved against the police when one of theirs was shot," said Amira el-Alfi, 33. "It is pretending, and is moving neither to protect institutions, nor to protect people."
On Monday, soldiers and military vehicles escorted thousands marching in the funeral procession for the civilians killed in the fighting the day before.
"It is now war between us and you, Interior Ministry," the marchers chanted as they carried the coffins of the dead to the cemetery. Many wave the black-white-and-green flags of Port Said that have become a symbol of the city's revolt against the government.
On their way back from the funeral, protesters hurled rocks at the police headquarters, which lies in a government complex. Police responded with volleys of tear gas. The military largely stood by. Flames were visible from the provincial government's headquarters and a nearby tax authority office, adjacent to the police building.
On Monday, both the military and the Interior Ministry, which is in charge of the police, sought to dispel any sign of friction between their forces. The Interior Ministry issued a statement saying "unknown elements" fired arbitrarily at the police and military with the aim of sowing sedition and causing escalation.
In a statement late Sunday, the military denied it fired at police. On Monday, the military spokesman Ali said that the fact that people on both sides were injured indicates that unknown elements were behind the gunfire.