Egypt's Hosni Mubarak was being kept alive by life support after the 84-year-old ousted leader suffered a stroke in prison Tuesday, officials said, deepening the country's uncertainty just as a potentially explosive fight opened over who will succeed him, with both candidates claiming to have won last weekend's presidential election.

The developments, which saw Mubarak moved out of prison to a military hospital, add further layers to what is threatening to become a new chapter of unrest and political power struggles in Egypt, 16 months after Mubarak was ousted by a popular uprising demanding democracy.

The Muslim Brotherhood, emboldened by its claims that it candidate won the election, sent tens of thousands of its supporters into the street. It was an escalation of its confrontation against the ruling generals over their grab this week of sweeping powers that give them dominance over the next president.

Some 50,000 protesters, mostly Islamists, protested in Cairo's Tahrir Square on Tuesday evening chanting slogans in support of the Brotherhood's candidate Mohammed Morsi and denouncing the generals.

“We, the people, gave them (the military) legitimacy and we now are taking back,” said Saber Ibrahim, a 36-year-old school teacher who came from his native Beni Suef south of Cairo to participate in the rally.

The conflicting claims over the election could further stoke the heat. The campaign of Mubarak's former prime minister, Ahmed Shafiq, said Tuesday he won the election, denying the Brotherhood's claim of Morsi's victory. Hundreds of his supporters took to the streets in Cairo in celebration.

The election commission is to announce the official final results on Thursday and no matter who it names as victor, his rival is likely to reject the result as a fraud. If Shafiq is declared winner in particular, it could spark an explosive backlash from the Brotherhood, which has said Shafiq could only win by fraud.

The sudden health crisis of Mubarak, who is serving a life prison sentence, briefly overshadowed the political standoff.

Moving Mubarak out of prison is likely to further infuriate many in the public. Many Egyptians have been skeptical of earlier reports that his health was worsening since he was put in prison on June 2, believing the reports were just a pretext to move him to another facility. There is a widespread suspicion that security and military officials sympathetic to their old boss are giving him preferential treatment.

Details of the crisis were still sketchy. Earlier the state news agency MENA and officials said that while at the Torah Prison hospital he suffered a “fast deterioration of his health.” His heart stopped beating until he was revived by defibrillation, then he suffered a stroke.

At that point, he was moved from the prison hospital to Maadi military hospital _ notably the same one where his predecessor Anwar Sadat was declared dead more than 30 years ago after being gunned down by Islamic militants.

When Mubarak arrived at the hospital, he was “clinically dead,” MENA reported. It said doctors repeatedly defibrillated him with no initial response. But later, a security official said Mubarak was on life support. The official, who spoke on condition of anonymity because he was not authorized to talk to the press, had no further details.

Maj. Gen. Mohsen el-Fangari, a member of the ruling military council, told the Al-Shorouk newspaper website that Mubarak was in a “very critical condition,” but denied he was dead. Mubarak's wife, Suzanne, came to the hospital, where Mubarak was in an intensive care unit, another security official said.

The criteria for using the term “clinically dead” are “poorly defined,” said Dr. Lance Becker, a University of Pennsylvania emergency medicine specialist and an American Heart Association spokesman.

“My speculation would be that he had that sort of event where his heart temporarily stopped,” said Becker, who is not involved in Mubarak's treatment.  “That doesn't mean that it's irreversible.” Life support can be used to keep his blood circulating and replace breathing if he is unable to do so on his own, Becker said.

Mubarak's condition brought to mind former Israeli Prime Minister Ariel Sharon _ though it was not known if there was any medical similarity in their conditions. Sharon suffered a massive stroke on 2006. Intensive treatment and repeated operations by a team of brain surgeons stabilized his condition, but he never regained consciousness. Sharon, 84, is still alive but remains on life support in a deep coma.

Mubarak has been serving a life sentence at Cairo's Torah Prison for failing to stop the killing of protesters during the 18-day uprising against his rule last year. The verdict against him has already been a spark for protests _ thousands massed in Tahrir when the court acquitted him and his sons on separate corruption charges and cleared several top security chiefs on the protester killings.

The multiple disputes have turned a moment that was once anticipated by some as a landmark in Egypt's post-Mubarak transition _ the election of the first civilian president in 60 years _ into a potentially destabilizing snarl.

Shafiq's campaign spokesman, Ahmed Sarhan, told a televised news conference that Shafiq won 51.5 percent of the vote and that the claim of victory by Morsi was “false.”

“Gen. Ahmed Shafiq is the next president of Egypt,” said Sarhan. He said Shafiq won some 500,000 votes more than Morsi, of the fundamentalist Brotherhood.

The Shafiq campaign's claim came just hours after Morsi's campaign repeated their claims of victory, saying Morsi had won 52 percent of the vote compared to Shafiq's 48.

The Brotherhood first announced Morsi's victory early Monday, around six hours after polls closed. It said its claim was based on returns announced by election officials from each counting center around the country. Each campaign has representatives at every center, who compile the individual returns. The Brotherhood's compilation during the first round of voting last month proved generally accurate.

Shafiq, a former air force commander who was named prime minister during Mubarak's last days, is seen by his opponents as likely to preserve the military-backed police state that his former boss headed for three decades. He, in turn, has presented himself as a strongman able to keep Egypt stable and out of the hands of the Brotherhood, playing on fears the group will turn the country into an Islamic state.

Just as polls closed on Sunday night, the military _ which has ruled since Mubarak fell on Feb. 11, 2011 _ issued a constitutional declaration that gave themselves power that all but subordinates the new president. Critics called it a “coup” intended to maintain their control over the state even after they nominally transfer authorities to the president by July 1.

The declaration gave the generals legislative powers and control over the process of drafting a new constitution and the national budget. It also shields the military against any kind of civilian oversight and allows the generals to run their own affairs without interference from civilian authorities.

A court ruling also dissolved the Islamist-dominated parliament last week, a verdict that has been endorsed by a decree issued by military ruler Field Marshal Hussein Tantawi. Also last week, the military-backed government granted military police and intelligence agents the right to arrest civilians for a host of suspected crimes, a move that many viewed as tantamount to a declaration of martial law.

The Brotherhood and its Islamist allies rejected the dissolution decree and insists the parliament is still in effect.

Tens of thousands demonstrated in Cairo and the Mediterranean port city of Alexandria Tuesday evening to denounce the constitutional declaration, which also strips the next president of significant powers and the court ruling.

The estimated 50,000 protesters in Cairo's Tahrir Square, birthplace of last year's anti-Mubarak uprising, were mostly Brotherhood supporters and other Islamists joined by a small group of leftist and liberal activists.

“It is not possible to have a revolution and then have military rule and a president with no authority,” said protester Mohammed Abdel-Hameed, a 48-year-old schoolmaster who said he came with his son and others to Tahrir from Fayyoum, an oasis province 60 miles (100 kilometers) southwest of Cairo.

“If they want blood we are ready to offer blood. I am. So that my son can live free,” warned Abdel-Hameed, who wore traditional galabeya robes.

Some protesters gathered outside parliament earlier Tuesday, and a number of lawmakers tried to enter the building but were turned away by security forces. Hundreds of black-clad policemen armed with clubs and shields ringed the building, standing behind metal barricades.

“Military rule again, no way! We're the power, we're the people,” protesters chanted. They addressed Tantawi, Mubarak's defense minister for 20 years, shouting, “You coward Field Marshal, free the parliament.”

The military's assertion of authority came under international criticism, from Amnesty International and former U.S. President Jimmy Carter, who has met repeatedly with the generals in visits to Egypt.

Carter said in a statement that he was “deeply troubled by the undemocratic turn that Egypt's transition has taken.” His Carter Center monitored the weekend runoff as it has every nationwide vote in Egypt since Mubarak's ouster in a popular uprising engineered by pro-democracy youth groups.

He pointed to the dissolution of parliament and the elements of martial law and said the constitutional declaration “violated the military's commitment to make a full transfer of power to an elected civilian government.

“An unelected military body should not interfere in the constitution drafting process,” said Carter. (AP)