Meanwhile, Mubarak, 84, suffered what state television called a "health crisis," and the BBC reported he was admitted to the hospital at Tora prison.
Huge crowds turned out at Tahrir Square, where demonstrators had gathered to protest against Mubarak last year, the BBC reported.
Opponents of the former government said they feared the life sentences given Mubarak and ex-Interior Minister Habib al-Adly for complicity in the deaths of protesters could be overturned on appeal.
Protesters expressed anger over the acquittals of four high-ranking ministry chiefs who had been accused of complicity in the protesters' deaths and of two regional security chiefs.
The acquittals, protesters said, indicate there has been little reform.
Some protesters expressed disappointment over the choice they now face in the presidential election in which Muslim Brotherhood candidate Mohammed Mursi faces Mubarak's last prime minister, Ahmed Shafiq.
Throughout much of his trial, Mubarak had suffered regular health lapses, The Daily Telegraph reported.
"Hosni Mubarak suffered from a surprise health crisis when his aircraft landed in Tora prison," the official MENA news agency reported.
Nile News, a state-owned news channel, said Mubarak suffered a heart attack, which had not been confirmed.
Meanwhile, Mubarak's lawyers said they would appeal the sentence.
The trial, the first of a former leader tried in person since the start of last year's Arab Spring that swept Mubarak from office after nearly 30 years in power, began in August and was adjourned Feb. 22, when the judge declared a verdict would be handed down Saturday.
Mubarak, who was present in the courtroom during sentencing, has denied any role in the killings.
Reaction in the courtroom at the Police Academy in the Fifth Settlement on the eastern outskirts of Cairo ranged from jubilant shouts to scuffles between Central Security Services forces and families of some of the 850 people killed during the demonstrations, The New York Times reported. Some chanted, "We got our retribution." Others demanded Mubarak and Adly be executed.
The killings of protesters in Tahrir Square and elsewhere occurred during the 18-day uprising that led to Mubarak's ouster.
Mubarak and his two sons, Gamal and Alaa, were acquitted of corruption charges.
During a long opening statement before the sentencing, presiding Judge Ahmed Refaat said the 10-month trial had been fair and spoke of 30 years of "darkness" the country had suffered under Mubarak's regime.