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.

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".

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".

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• April 1975

  Egyptian President Anwar Sadat choses Hosni Mubarak as his vice president.

• Oct. 14, 1981

  Following Sadat's assassination during a military parade, Mubarak is sworn in as president.

• June 26, 1995

  The Egyptian president survives an assassination attempt while visiting Ethiopia.

• February 2005

  Mubarak orders constitutional amendments that pave the way for Egypt's first-ever multi-candidate elections.

• March 2005

  Hundreds participate in protests to oppose a fifth term for Mubarak or plans to let his son Gamal succeed him.

• Sept. 7, 2005

  Egypt holds its first multi-candidate presidential election is held. Mubarak wins easily over nine other candidates and is sworn in for a fifth consecutive presidential term several weeks later.

• Nov. 28, 2010

  Egypt holds parliamentary elections, but opposition parties claim the elections are marred by fraud.

• Jan. 25, 2011

  Thousands of anti-government protesters in Cairo stage the biggest demonstrations in Egypt in years.

• Jan. 28, 2011

  Mass protests rock Cairo and other cities. Mubarak orders the army out on the streets to restore order.

• Jan. 29, 2011

  Mubarak appoints his first vice president, Omar Suleiman in a bid to appease protesters. A wave of looting grips Egypt after police withdraw from Cairo and several cities.

• Feb. 10, 2011

  Mubarak refuses to leave office but hands some powers to Vice President Suleiman.

• Feb. 11, 2011

  Mubarak resigns and hands power to the military after protesters flood the streets of Cairo and other cities.

• April 13, 2011

  Mubarak is detained along with his two sons for ordering the use of lethal force against protesters and corruption. The elder Mubarak is held in custody at a Sharm el-Sheikh hospital to monitor his health.

• July 14, 2011

  Mubarak denies responsibility for the killing some 850 protesters by his security forces, during Egypt's uprising, according to a published transcript of his interrogation.

• June 2, 2012

  Hosni Mubarak is sentenced to life in prison for acting as an accomplice in the killing of demonstrators during the January 2011 protests. (AP Photo, File)