Beside the 21 death sentences, the court handed down life sentences - which in Egypt are equal to 25 years - to four defendants, and 15-year-sentences to three others, MENA said.

The 28 persons were charged with belonging to an outlawed group linked to ISIS, disrupting public order, possession of weapons and endangering society among other charges. Of the total, only 12 are in custody while the others - 16 suspects - are at large.

Rights groups have repeatedly criticised similar mass sentencings in Egypt and called on authorities to ensure fair trials.

Egyptian authorities have been carrying out a wide-ranging crackdown on dissent since the 2013 military overthrow of Islamist President Mohammed Morsi, arresting thousands of his supporters as well as some well-known secular activists.

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Earlier on Thursday, six suspected members of the now-outlawed Muslim Brotherhood group were ordered to be detained for 15 days, pending investigation over joining a "terrorist" group.

The six were arrested on Wednesday night on a farm allegedly belonging to detained former presidential candidate and "Strong Egypt" party leader, Abdel-Monaem Abul Fetouh. He was a longtime Brotherhood member before he quit the group in 2011.

Egypt listed Abul Fetouh on its terrorism list earlier this week, a designation that bans those on the list from travel, puts them on a watch list and grants authorities the right to freeze their assets.

Over the past years, Egyptian authorities have put the names of hundreds, including senior Muslim Brotherhood figures, on the list.

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