Update: Mubarak has now been taken off life support, according to Sky News.
Former Egyptian president Hosni Mubarak is being kept alive by life support after he was rushed from a prison hospital to a military one in a rapidly worsening condition, officials said.
The 84-year-old ousted leader's health crisis added a new element of uncertainty just as a potentially explosive fight opened over who will succeed him.
The state news agency Mena said Mubarak was "clinically dead" when he arrived at the hospital and that doctors used a defibrillator on him several times. It initially said the efforts were not successful.
But the official said Mubarak was put on life support. He had no further details on his condition.
The developments 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. Egyptians were uncertain about Mubarak's fate, about who will succeed him and about whether his successor will have any power.
Hosni Mubarak is said to have had a stroke
The campaign of Mubarak's former prime minister, Ahmed Shafiq, said on Tuesday he has won Egypt's presidential election, countering the Muslim Brotherhood's claim of victory for its candidate, Mohammed Morsi. 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.
The Brotherhood, Egypt's most powerful political group, is already escalating its challenge against the ruling military over the generals' move this week to give themselves overwhelming authority over the next president.
On Tuesday night thousands of activists gathered in Tahrir Square in a rally called by the Muslim Brotherhood in protest against the fresh powers awarded to the military.
Egypt's ruling military council has granted itself wide-reaching new powers. Tension is already brewing after the supreme court dissolved parliament last week. The Supreme Council of Armed Forces (Scaf) has given itself full legislative control, insisting that new parliamentary elections cannot be held until a constitution is agreed. On Twitter, former UN diplomat Mohamed ElBaradei described the move as a "setback for democracy".
Muslim Brotherhood supporters wave national flags and chant slogans, and one carries a banner that reads in Arabic "The law of the revolution not the law military" on Tuesday night
The health crisis of Mubarak, who is serving a life prison sentence, is yet one more thing to stoke the heat.
Moving Mubarak out of prison is likely to further infuriate many. A number of Egyptians have been sceptical of earlier reports that his health was worsening since he was jailed 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 remain sketchy. Earlier the news agency 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. That was when Mena reported him "clinically dead".