At least 49 people have been killed in clashes in Egypt as the country marks the anniversary of the 2011 uprising which overthrew President Hosni Mubarak, the health ministry says.
Rival demonstrations of supporters and opponents of the military-backed government took place in Cairo.
But police broke up anti-government protests, and arrests were reported in Cairo and Alexandria.
Hundreds have died since July when the army deposed President Mohammed Morsi.
Extra security measures were in place for Saturday.
Flags and banners
Egyptian Interior Minister Muhammad Ibrahim had urged Egyptians not to be afraid to go to events marking the anniversary of the uprising.
Thousands of supporters of the military and the government gathered in high-profile locations including Tahrir Square - the focal point of the 18-day 2011 popular revolt.
Participants waved Egyptian flags and banners showing army chief Gen Abdel Fattah al-Sisi, whom many urged to run for president.
Meanwhile on Saturday, an army helicopter crashed in the restive Sinai peninsula, with an unconfirmed report that its crew of five soldiers was dead.
A large car bomb exploded near a police building in Suez, at the southern entrance of the Suez canal, with reports that nine people were injured.
At least 18 people died in violence on Friday.
The BBC's Yolande Knell, in Cairo, says that three years on from an uprising that raised hopes of political reform in the Arab world's most populated country, rival demonstrations are showing the deep divisions.
There is an extreme anti-Islamist emphasis at pro-government rallies, with chants for "the execution of the Brotherhood" and fury at anyone believed to be critical of the post-coup leadership, reports said.
At anti-government protests, police chase protesters into side streets, firing live rounds as well as tear gas and birdshot.
One of those killed was a member of the April 6 movement, which led protests against Mubarak before and during the 2011 uprising and also opposed Mr Morsi, the group said.
Scores of arrests have been reported in Cairo and Egypt's second city, Alexandria - not just of Islamist supporters of deposed President Morsi, but secular opponents of the military government who have also been protesting.
"The only thing allowed is Sisi revolutionaries," one of the activists, blogger Wael Khalil, told the Associated Press news agency.
"This was supposed to be a day to mark the revolution... I don't get it. Do they think that there will be a working democracy this way?"
Al-Jazeera journalist Peter Greste - detained by Egyptian authorities for nearly a month - has written a letter from solitary confinement, describing Egypt's prisons as "overflowing with anyone who opposes or challenges the government".
The Anti-Coup Alliance, led by Mr Morsi's Muslim Brotherhood, called in a statement for 18 days of protests beginning on Saturday, mirroring the 18 days of protests that three years ago led to Mr Mubarak stepping down.
The Brotherhood has regularly held protests since the overthrow of Mr Morsi. Hundreds of its supporters have been killed, and thousands detained.
It has been declared a "terrorist organisation" and accused by the interim government of being behind a string of violent attacks in recent months, which the Brotherhood denies.
In a defiant statement on Saturday, the Brotherhood vowed not to leave the streets "until it fully regains its rights and breaks the coup and puts the killers on trial", reported the Associated Press news agency.