David D. Kirkpatrick and Kareem Fahim / New York Times News Service

CAIRO — Former President Hosni Mubarak, who led Egypt for three decades until he was toppled last year in a popular uprising, was on life support at a military hospital late Tuesday after he was declared “clinically dead” by doctors, according to Egyptian officials and state news media.

After suffering a stroke in prison Tuesday, Mubarak, 84, was moved to a military hospital overlooking the Nile in Cairo. Doctors said they were unable to revive him after he went into cardiac arrest, state news media reported.

News of Mubarak’s failing health quickly spread through crowds of demonstrators in Tahrir Square, adding a new element of volatility to the growing political and constitutional crisis in the country he once ruled.

The reports came as elements of the government that Mubarak had led tried to re-establish their power and as hundreds of thousands of people were protesting in the streets, trying to salvage the spirit of their revolt. Mubarak was last seen in public 15 days ago when he was given a life sentence for collaborating in the killing of demonstrators during protests that ended his iron-fisted rule.

His health deteriorated rapidly when he was flown by helicopter from the courthouse to a hospital ward in a notorious prison where many political prisoners were held during his rule.

It was unclear what effect the news of Mubarak’s condition might have on the political standoff that has followed the nation’s first competitive presidential election. According to a public ballot count confirmed by official media, Mohammed Morsi, the Brotherhood’s presidential candidate, defeated Ahmed Shafiq, a former general who once served under Mubarak in the air force and was his last prime minister.

The day before voting began, the generals who have ruled Egypt since Mubarak left power seized on a rushed decision by a court of Mubarak-appointed judges to shut down the Brotherhood-led Parliament. They have proceeded to issue their own interim constitution that entrenches their power while all but eviscerating the authority of the new president.

As Mubarak lay in a hospital on Tuesday night, tens of thousands of demonstrators filled central Cairo — the biggest stage for the revolution that removed Mubarak — to demand that the generals back down. The Brotherhood bused in thousands of supporters and vowed to push a three-pronged campaign of street protests, legal battles and political negotiations until their control of Parliament and the presidency was restored.