Egypt sentences 21 to death on 'terrorism' charges
Sixteen of the 21 sentenced to death were tried in absentia, Reuters reported.
The court also charged four others with life sentences - capped at 25 years in Egypt - and another three with 15-year jail terms.
Thursday's verdict can still be appealed within 60 days before the court of cassation, Egypt's highest appellate court.
Earlier in the day, six suspected members of the Muslim Brotherhood were detained for 15 days, pending investigation over joining a "terrorist" group.
The Muslim Brotherhood is outlawed in Egypt.
The six were arrested on a farm allegedly belonging to Abdel-Monaem Abul Fetouh, a former presidential candidate now in jail.
Egypt has been carrying out a wide-ranging crackdown on dissent since the 2013 military overthrow of President Mohamed Morsi, a leading Brotherhood figure.
Hundreds of Islamists have received death sentences since then, with authorities carrying out dozens of executions.
Earlier this month, the European Parliament called on Egypt to "halt all imminent executions" and condemned the government for its frequent use of the death penalty.