CAIRO (AFP) - An Egyptian appeals court on Monday upheld three-year prison sentences for three prominent activists charged with violating a controversial law restricting protests.

The three, including the founder of the April 6 movement Ahmed Maher, rose to prominence in the 2011 uprising that toppled veteran strongman Hosni Mubarak.

The interim government installed after the army overthrew Islamist president Mohamed Mursi in July jailed the activists for violating a law it had passed banning all but police-sanctioned protests.

Maher, Mohamed Adel and Ahmed Douma were charged with organising an unauthorised and violent protest in November, days after the passage of the law.

The sentencing in December of the activists, members of a broad coalition of groups that supported Mursi's ouster, had raised concerns of a return to Mubarak-era repression under the new military-installed regime.

The former army chief who overthrew Mursi, General Abdel Fattah al-Sisi, is running for president next month. He has promised there will be "no return" to the practices of Mubarak's regime, which was toppled by an Arab Spring uprising after ruling Egypt for three decades.

The retired field marshal is expected to easily win the May 26 to 27 vote as he is riding a wave of popularity for ending Mursi's divisive year in power.

Secular-leaning activists have increasingly been targeted in an extensive crackdown on the opposition, mainly Islamists, that has seen an estimated 15,000 people jailed.

Another prominent left-wing activist, Alaa Abdel Fattah, is standing trial on similar charges to those the trio faced.

Mr Ahmed Seif, a lawyer for Maher and his co-defendants, said they will appeal the ruling and, if that fails, will take the case to the African Court of Human and People's Rights.

"There are no indications that the state is willing to loosen its grip," Mr Seif told AFP after the ruling.

Human Rights Watch criticised Monday's verdict.

"Today's verdict against three of the most recognised faces of the Jan 25, 2011 protests is one more nail in the coffin for Egypt's revolution," Ms Sarah Leah Whitson, HRW's director for the Middle East and North Africa, said in a statement.

Critics say the government has given police a free hand to clamp down on dissent, amid a crackdown that has seen more than 1,400 people killed in street clashes since Mursi's overthrow.

Courts have tried hundreds of Islamists en masse, with one court sentencing 529 to death for allegedly participating in a deadly riot. That sentencing is likely to be overturned on appeal.

Militants have meanwhile unleashed a deadly campaign against security forces that has killed nearly 500 policemen and soldiers in bombings and shootings since Morsi's ouster.

The government said last week it had boosted punishment for "terrorist" offences and expanded the scope of the crimes that fall under that category.

The legal amendments will come in force when interim president Adly Mansour approves the government-penned draft.