An Egyptian court on Wednesday convicted a prominent activist from the 2011 uprising of organizing an unauthorized protest and assaulting a policeman, sentencing him to 15 years in prison, in the latest blow to liberal activists at a time of rapidly eroding freedoms.

The sentence against Alaa Abdel-Fattah is the toughest against any of the secular activists behind the 18-day uprising that ended Hosni Mubarak’s 29-year reign. It is also the first conviction of a prominent activist since former army chief Abdel-Fattah el-Sissi took office as president Sunday.

In the 11 months since el-Sissi ousted the country’s first freely elected president, the Muslim Brotherhood’s Mohammed Morsi, authorities have launched a massive crackdown on Islamists, detaining at least 16,000 and killing hundreds. Secular activists opposed to what they see as the revival of Mubarak’s police state have also been detained.

The crackdown is being carried out in the face of a burgeoning insurgency by Islamic militants, who have killed and wounded hundreds of policemen and army troops since Morsi’s ouster.

As the government has moved to curb freedoms won in the 2011 revolt — including a new law that severely restricts protests — pro-military media have stoked a resurgent nationalism and eagerly welcomed the return of a military man to the presidential palace.

Security officials said that while Abdel-Fattah was convicted and sentenced in absentia, he did turn up at the Cairo courtroom later on Wednesday and was detained by police. The absentia sentencing means that he now faces an automatic retrial, although the conviction stands in the meantime.

A human rights lawyer and family members said that the judge opened the proceedings earlier than scheduled and that Abdel-Fattah was kept waiting outside the courthouse, at a police academy south of Cairo, as he sought permission from the judge to enter the heavily guarded complex.

Abdel-Fattah was accused of inciting an “unauthorized” demonstration on Nov. 26. Protesters were angry about a clause in the draft of a new constitution that allows military trials for civilians. The constitution was later adopted by referendum.

Mona Seif, Abdel-Fattah’s sister and one of the organizers of the Nov. 26 protest, said her brother attended the demonstration but denied he had organized it.

The demonstration was broken up by police on the grounds that organizers had no permit. Female participants, including Seif, were snatched by police and thrown into a van before being dumped in the middle of the desert that night.

Two other leading activists from the 2011 uprising, Ahmed Maher and Ahmed Doumah, are serving three-year sentences for their alleged part in the November protest.

“The verdicts are meant to exact revenge and send a message of intimidation to whoever dares to speak up against injustice. But the result will be more anger, not fear,” said prominent lawyer and rights activist Gamal Eid.

Prosecutors accused Abdel-Fattah of organizing an illegal demonstration and illegal possession of an object that could be used as a weapon. He and 24 other defendants are accused of using force to take a policeman’s two-way radio, wounding him in the process, blocking traffic and posing a threat to public safety and order.

The 24 were also convicted and sentenced to 15 years in jail. At least two of them were arrested with Abdel-Fattah.

El-Sissi has said that he intends to uphold the protest law and that freedom of speech will have to take a back seat to restoring security and reviving the nation’s ailing economy.