CAIRO — 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 of 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 on 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 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.

Abdel-Fattah was accused of inciting an "unauthorized" demonstration on Nov. 26 against a clause allowing military trials for civilians in the draft of a new constitution, which 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, saying it was called by a group that campaigns against military trials for civilians.

Police violently disbanded the demonstration 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 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 civil rights activist Gamal Eid.

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.

In Washington, U.S. State Department spokeswoman Jen Psaki said the protest law hindered democracy.

"We share the view of Egyptian civil society representatives that this law, which imposes restrictions on Egypt's ability to assemble peacefully and express their views, does not meet international standards and will not move Egypt's democratic transition forward. And as is the case in any of these circumstances we have, we will continue to express that to the relevant officials in Egypt," Psaki told reporters.

In his inauguration speech on Sunday, the 59-year-old el-Sissi said freedoms must be limited by "religious and moral principles" and that criticism must be objective and free of slander.

In a thinly veiled threat to activists, he said there would be zero tolerance for anyone who seeks to "disrupt our march toward the future."

An outspoken blogger, Abdel-Fattah has been in and out of prison in the three years since Mubarak's ouster. He campaigned against military trials for civilians during the 17 months that generals held power following Mubarak's resignation. He opposed Morsi but strongly disapproved of the military's return to politics.

Egyptian courts have been meting out unusually harsh sentences against Islamists and others in recent months, prompting many rights activists to raise questions about the judiciary's integrity.

Hundreds of Islamists have been sentenced to death, in one case after just two court hearings. There have also been a growing number of reports by rights groups of abuse, and torture in some cases.

Activists have meanwhile contrasted a recent string of what they see as lenient rulings for policemen accused of killing protesters and the heavy sentences given to anti-military activists.

This week, a Cairo appeals court canceled prison sentences passed against four policemen convicted of manslaughter in the case of 37 alleged Morsi supporters who suffocated to death inside a police vehicle on Aug. 18.

Also this week, a policeman convicted of torturing to death a fundamentalist Muslim was released pending a retrial. The man who died in custody, Sayed Belal, had been arrested over a 2010 church bombing in Alexandria, and the officer was initially sentenced to 15 years in prison.