Egypt carried out Saturday the first death sentence handed down over the violence that erupted after the army overthrew Islamist president Mohamed Morsi in 2013, the interior ministry said.
Hundreds of Morsi supporters have been sentenced to death after speedy mass trials, which the United Nations has described as "unprecedented in recent history".
Mahmoud Ramadan, who was hanged at 7 am (0500 GMT), was the "first to be executed of those involved in violent clashes," ministry spokesman Hani Abdel Latif told AFP.
A court in Egypt's second city Alexandria sentenced Ramadan to death in 2014 after convicting him of throwing youths off an apartment block, killing one of them.
He was among dozens of people tried over deadly violence in the city's Sidi Gaber neighbourhood on July 5, 2013, two days after Morsi's ouster by then army chief and now President Abdel Fattah al-Sisi.
"The prison authority carried out the hanging of Mahmoud Ramadan, accused of throwing children from the roof of a building in Alexandria during violent clashes organised by the terrorist Muslim Brotherhood," the interior ministry said on its Facebook page.
The unrest on that day came as Morsi supporters and opponents held rival demonstrations across the city.
Amateur video footage of the protests had gone viral on the Internet, with prosecutors using it as evidence in the case involving Ramadan.
In one scene, a bearded Ramadan in a white vest and black trousers is walking on the roof of a building.
Four youths are seen cowering on top of a structure there that appears to be a water tank as the man identified as Ramadan and others throw stones at them.
Minutes later two youngsters fall off the structure onto the roof.
In separate footage, aired by a private television channel after his alleged confession, Ramadan says: "When I appear before the prosecution, I will demand to be executed."
A government crackdown on Morsi's supporters killed hundreds, with thousands more arrested and tried.
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The ousted president is himself facing multiple trials on charges that carry the death penalty.
The leader of Morsi's Muslim Brotherhood movement, Mohamed Badie, has already been tried and handed down four life sentences in separate cases, while one death verdict was overturned on appeal.
- Mass sentences criticised-
Many of the prosecutions have been in mass trials, which then UN human rights chief Navi Pillay described last June as "obscene and a complete travesty of justice."
Rights groups and activists too have criticised the authorities, accusing them of using the judiciary as a tool of repression to crack down on Morsi supporters or on any kind of dissent.
"Mass death sentences are fast losing Egypt's judiciary whatever reputation for independence it once had," Sarah Leah Whitson, Human Rights Watch's Middle East and North Africa director, said in December after a court sentenced 188 Morsi supporters over the killing of 13 policemen.
The crackdown has triggered retaliatory attacks by Islamists targeting security forces in Cairo and other major cities, but mostly in the Sinai Peninsula, where troops are battling a raging insurgency.
Scores of policemen and soldiers have been killed in the attacks, most of them claimed by Ansar Beit al-Maqdis, the Egyptian affiliate of the Islamic State group.
Late Friday, a bomb blast in the textile town of Mahallah, north of Cairo, killed one person and wounded 11.
The interior minister who oversaw the crackdown on Morsi's supporters, Mohamed Ibrahim, was replaced Thursday amid mounting criticism of the failure by police to halt the attacks.
His successor, Magdy Abdel Ghaffar, appointed 25 new police commanders Friday in a major shake-up.