Egypt has carried out the first death sentence handed down over the violence that followed the overthrow of President Mohammed Morsi in 2013.
Mahmoud Ramadan, a supporter of the deposed leader and convicted of murder, was hanged.
The charge related to an incident when youths were thrown from a building in the city of Alexandria.
Hundreds of people have been sentenced to death in an Islamist crackdown following Mr Morsi's removal.
The violence in Alexandria's Sidi Gaber district broke out in the days after the army deposed Mr Morsi on 3 July 2013, following mass protests against his rule.
The state news agency Mena said that 18 people were killed in the clashes.
The Muslim Brotherhood has denied any involvement in the killings.
Footage of two youths being thrown from a roof in the Mediterranean city was widely broadcast in Egypt at the time.
One of them - Hamad Badr, who had just turned 19 - later died in hospital.
His father, Badr Hassouna, told the BBC that his son had been watching a pro-Morsi protest when gunfire broke out.
He and others ran into the apartment building, where they were chased onto and then thrown from the roof by the protesters.
Ramadan was sentenced to death a year ago.
Over the past 18 months, there have been speedy mass trials and a crackdown on the Muslim Brotherhood, which backed Mr Morsi.
The army chief-turned-President Abdel Fattah al-Sisi, who ousted Mr Morsi, has since banned the Islamist movement.
Ramadan's execution comes a month after Egypt's high court upheld his conviction, Reuters news agency reports.
Most of the other death sentences of alleged Muslim Brotherhood supporters are still under review, it says.
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