The accused boys were jailed by an Alexandria court for joining rallies demanding the return of the ousted Islamist president, Mohammed Morsi.
The boys are believed to be all under the age of 18.
"The 78 minors, members of the Muslim Brotherhood, were arrested for participating in protests organized by the group calling for the downfall of the regime in which they blocked roads and transportation, and terrified citizens," Egypt's state MENA news agency reported.
Their jailing is part of a long-running crackdown by the country's military-led authorities, targeting Morsi supporters who are calling for his reinstatement as president. Hundreds of Brotherhood members have been killed and thousands of its leaders and activists, including Morsi himself, remain in prison.
Morsi is Egypt's first elected and civilian president and was removed from office by the army in July 2013, following mass protests against his short rule. His ouster sparked widespread conflict across the country.
Morsi is currently on trial in four separate cases, including one where he stands accused of inciting the killing of opposition protestors during his time in office.
jr/jm (AFP, dpa)