An Egyptian emergency court on Sunday sentenced former presidential candidate Abdelmoneim Aboul Fotouh to 15 years in prison for "spreading false news" and "incitement against state institutions", judiciary sources said.
Aboul Fotouh has been in detention since 2018 and his trial began last November, though Egyptian law sets a legal cap of two years for pretrial detention.
The verdict cannot be appealed.
He was sentenced alongside 24 others, including members of the Muslim Brotherhood group, from which the former candidate had defected years before.
Also receiving a 15-year sentence was the Brotherhood's former supreme guide Mahmoud Ezzat, who is already serving multiple life sentences on other charges.
Mohamed al-Qassas, the deputy head of Aboul Fotouh's Strong Egypt party, received a 10-year sentence, while the remaining defendants were sentenced to between 10 years and life in prison.
Lawyer Khaled Ali, a prominent opposition figure and himself a former presidential candidate, last month submitted what he said was irrefutable evidence of Aboul Fotouh's innocence.
Episodes of a TV series screened during the month of Ramadan -- featuring real-life footage of Aboul Fotouh criticising the Brotherhood, filmed by state intelligence without his knowledge -- were used in his defence.
Amnesty International condemned Sunday's ruling, describing the trial as "grossly unfair and politically-motivated" and alleging that the defendants had been "subjected to a litany of abuses including torture".
The rights group also said that Aboul Fotouh, aged 70, had been denied medical care for years.
In 2012, Aboul Fotouh was among a number of candidates who ran unsuccessfully in elections that saw the Muslim Brotherhood's Mohamed Morsi voted into power.
Morsi's divisive rule lasted for just one year, before he was ousted by then-defence minister Abdel Fattah al-Sisi following mass protests in July 2013.
The Brotherhood was subsequently declared a terrorist organisation and thousands of its members and sympathisers arrested in an ensuing crackdown.
Aboul Fotouh was arrested in 2018 after he joined a call to boycott that year's presidential election, in which Sisi was reelected with a landslide 97 percent of votes.
Once a leading member of the Muslim Brotherhood, the former candidate had supported the mass protests against the Islamists, before going on to become a critic of Sisi and his ensuing clampdown on dissent.
He was sacked from the Brotherhood's leadership in 2009, and has since denied any ongoing association with the group.
Aboul Fotouh's assets have been frozen since he was arrested and added to the state's terror list for "leading a terrorist organisation".