The swift conviction of so many in one stroke was a sudden acceleration of the sweeping crackdown against Mr. Morsi’s Islamist supporters and against other dissenters that has unfolded since his removal last summer. After the overthrowing of Mr. Morsi, the military-led government killed more than a thousand people in shootings during protests against the takeover, and since then it has arrested many thousands of others as demonstrations have continued at universities and in the streets. Most of those arrested have been detained without charges or trials.

The verdict on Monday underscored the continuing determination of at least a part of the Egyptian judicial system to treat support for the ousted president as treason. In December, the government formally outlawed the Muslim Brotherhood, the Islamist group that backed Mr. Morsi and dominated Egypt’s first free elections, declaring it a terrorist group and subjecting any of its members or supporters to heavy penalties. The government has also issued another law criminalizing participation in unauthorized street demonstrations, with jail sentences for organizers.