Alaa Abdel-Fattah

CAIRO — A court on Wednesday convicted a prominent activist from Egypt’s 2011 uprising for demonstrating without permit and assaulting a policeman, sentencing him to 15 years in prison.

The sentence against Alaa Abdel-Fattah is by far the toughest against any of the liberal, pro-democracy activists behind the 18-day uprising that toppled Hosni Mubarak’s 29-year regime. It is also the first conviction of a prominent activist since former army chief Abdel-Fattah El-Sisi took office as president on Sunday.

In the 11 months since El-Sisi 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. That crackdown has overshadowed another, albeit smaller, campaign against secular activists opposed to what they see as the return of Mubarak-era policies.

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.

The case against Abdel-Fattah dates back to late last year when he was accused of taking part in an “unauthorized” demonstration against a controversial law that places rigid restrictions on street protests.

According to prosecutors, Abdel-Fattah was accused of taking part in an illegal demonstration, using force to take possession of a two-way radio held by a policeman and blocking traffic. Twenty-four defendants in the same case were also convicted and sentenced to 15 years in jail in absentia.

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

In his inauguration speech on Sunday, he said freedoms must be structured within “religious and moral principles” and criticism must be objective and free of slander.

Abdel-Fattah, an outspoken blogger, has been in and out of prison in the three years since Mubarak’s ouster. He and his activist sister, Mona Seif, were leading figures in the protest movement that forced Mubarak to step down in 2011 and both have vigorously campaigned against military trials for civilians under the rule of the generals who took over from Mubarak and ruled Egypt for nearly 17 months.