Angry crowds have gathered in Cairo's Tahrir Square, hours after ex-President Hosni Mubarak was jailed for life for his part in the killing of protesters during Egypt's 2011 revolution.
The crowds are furious at the acquittal of key security officials who were on trial alongside Mubarak.
Four interior ministry officials and two local security chiefs were cleared of complicity in protesters' killings.
Rallies against the verdict were also held in Alexandria, Suez and Mansoura.
Correspondents say a verdict that was meant to bring closure for Egypt is in danger of reopening old wounds.
In another development, dozens of protesters stormed the campaign headquarters of presidential candidate Ahmed Shafiq in the Fayoum area south of Cairo, Egyptian media reported.
Mr Shafiq was Mubarak's last prime minister.
Verdict 'mocks us'
Some protesters at Tahrir Square, the focal point of last year's uprising, say they are determined to begin a sit-in.
They have been joined by prominent public figures and football fans known as Ultras, who have been implicated in a number of political confrontations.
The slogan from last year's uprising: "Down with the military rule" is being chanted in the square and many have vociferously condemned Saturday's verdict.
The BBC's Yolande Knell, at the square, says there is particular anger at the acquittals of the officials, which many take as a sign that there has been little reform.
"The Mubarak verdict mocks us. He and [former Interior Minister Habib] Adly got a sentence and their aides got nothing," protester Sharif Ali told the BBC. "When they return to court on appeal, they will be freed too."
But, our correspondent adds, others have poured on to the streets out of depression at the current political situation.
The first round of recent presidential elections has left Egyptians with a choice between an Islamist candidate and an ex-prime minister from the Mubarak era.
Scuffles in court
The 84-year-old former president is the first former leader to be tried in person since the start of the Arab Spring in early 2011.
Announcing the verdicts, Judge Ahmed Refaat said Mubarak and former Interior Minister Habib al-Adly had failed to stop security forces using deadly force against unarmed demonstrators. They were both given life terms.
The judge insisted that the 10-month trial had been fair.
He said the Mubarak era had been "30 years of darkness" and praised what he called "the sons of the nation who rose up peacefully for freedom and justice".
Mubarak and his two sons, Alaa and Gamal, were acquitted on separate charges of corruption. But his sons will remain in detention as they are to be charged with stock-market manipulation.
After the verdict, scuffles erupted in court. Outside, sentencing was initially greeted by celebrations, but anger soon took over when news of the acquittals spread.
State television reported that as he was being transferred to jail, Mubarak at first refused to leave the helicopter and then suffered from severe health problems. He has reportedly been admitted to the prison hospital.
Mubarak, who ruled the country from 1981 to 2011, had faced a possible death sentence over the killing of about 850 protesters.
The first leader toppled during the Arab Spring was Zine al-Abidine Ben Ali of Tunisia, who was found guilty in absentia of drugs and gun charges in July.
Libyan leader Muammar Gaddafi was killed by rebels in October. Yemen's Ali Abdullah Saleh received immunity from prosecution after handing over power in November.