A military court in Cairo on Sunday condemned eight people to death for plotting violence and assassinations on behalf of the Muslim Brotherhood, and passed lengthy prison sentences on another 18.
The verdict comes almost a year after the military released a video showing a number of the defendants making confessions, amid allegations from relatives that they had been secretly detained and tortured.
Twelve of the defendants received life sentences and six were jailed for 12 years, while two were cleared. All the sentences can be appealed to a military appeals court.
Amnesty International condemned the verdict as an "affront to justice."
"Sentencing to death men who were tortured into 'confessions' is an egregious injustice, even by the degraded standards of Egypt's justice system," said Magdalena Mughrabi-Talhami, Amnesty deputy programme director for the Middle East.
Among those sentenced to life imprisonment was Suhaib Saad, who was jailed alongside three Al Jazeera journalists in 2014 in a high profile trial that drew international criticism over Egypt's crackdown on the media.
The journalists, accused of publishing false news and collaborating with the banned Brotherhood, were pardoned by President Abdul-Fattah al-Sissi last September.
Family and friends of Saad and a second defendant, Omar Mohammad, had mounted a high-profile social media campaign after they and a third friend went missing while out for dinner in June. Saad was out on bail pending a retrial in the Al Jazeera case at the time.
Saad's brother Osama told dpa that the two turned up in prison two weeks later and that Saad said he had been tortured throughout that period, including being hung up by the wrists and given electric shocks.
Amnesty quoted lawyers and relatives in the case as saying that defendants had been held incommunicado and tortured in military intelligence detention centres, under conditions of enforced disappearance, for up to 46 days.
In July, the Defence Ministry published a video announcing that security agencies had captured "one of the most dangerous terrorist cells" seeking to threaten the country's security.
Saad and other defendants in the case were shown confessing to forming an underground Brotherhood cell taking orders from exiled leaders of the Islamist movement.
Saad's lawyer at the time, Yasmin Hossam al-Din, told dpa that he was "a purely peaceful political activist."
Egyptian courts have sentenced hundreds of people to death for involvement in Islamist violence since al-Sissi, then head of the armed forces, ousted Islamist president Mohammed Morsi in 2013.
Almost all those death sentences, including one passed on Morsi himself, remain subject to appeal, with only seven known to have been carried out.
In 2014 al-Sissi issued a law expanding the jurisdictions of military courts. Human Rights Watch in April said that over 7,000 civilians had undergone military trials since then.
The New York-based rights group said the courts' proceedings were "neither fair nor credible." Egyptian authorities say proceedings in military courts afford defendants the same protections as civil courts but can deal with cases quicker.