The authorities have engaged in a crackdown on Mohamed Morsi's supporters since the army deposed him last year, with hundreds jailed in mass trials the United Nations has described as "unprecedented in recent history".
On Wednesday a court in the Mediterranean city of Alexandria sentenced 78 boys who are under 18 for joining the blacklisted Muslim Brotherhood's rallies in the past three months, a judicial official said.
While the judicial sources said the boys were aged between 13 and 17, their defence lawyer said the youngest was 15.
"The 78 minors, members of the Muslim Brotherhood, were arrested for participating in protests organised by the group calling for the downfall of the regime in which they blocked roads and transportation, and terrified citizens," state agency MENA reported.
They will be held in juvenile detention until they turn 18, when they will be transferred to adult prison.
Last December, the government declared the Muslim Brotherhood a "terrorist organisation".
"The number of children jailed since Morsi's ouster is unprecedented," said Ahmed Messilhy, head of a committee to defend children at Egypt's lawyers syndicate.
Defence lawyer Ayman El Dabi said he planned to appeal the ruling within the next few weeks.
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The defendants, he said, were arrested at several protests over the past year and had been in detention since.
Some were not even protesting and "were in the wrong place," he added.
Since Morsi's ouster, the government has launched a crackdown against his supporters that has left at least 1,400 people dead and more than 15,000 behind bars.
A number of Brotherhood leaders, including Morsi himself, are facing trials in several cases that might result in death sentences.
Dozens of Islamists have already been sentenced to death in mass trials.
Morsi was overthrown after millions took to the streets demanding his resignation after just one year in office, accusing him of monopolising power and ruining an already weak economy.
The authorities accuse the Brotherhood of planning and carrying out deadly attacks targeting security forces since Morsi's ouster. The movement has regularly condemned these attacks.
These attacks have been claimed by jihadist groups who say they are in retaliation for the crackdown on Islamists.
The most active such group, Ansar Beit al-Maqdis, which claimed an attack that killed 30 soldiers on October 24 in North Sinai, recently pledged allegiance to the Islamic State group, which has seized swathes of territory in Iraq and Syria.