More than 500 backers of Egypt's blacklisted Muslim Brotherhood were arrested as clashes erupted on the anniversary of its 2011 uprising, a minister said Monday, in the biggest police sweep for months.
Twenty people, mostly demonstrators, were killed Sunday when protesters clashed with security forces after Islamists called for rallies against President Abdel Fattah al-Sisi's government as Egypt marked the fourth anniversary of the toppling of ex-strongman Hosni Mubarak.
Supporters of Mubarak's successor, Islamist Mohamed Morsi, have regularly clashed with security forces since he was ousted by then army chief Sisi in July 2013.
Rights groups have repeatedly denounced the use of "excessive force" by the authorities to crush opposition rallies.
"We arrested 516 elements from the Muslim Brotherhood group who were involved in firing ammunition, planting explosives and bombing some facilities" on Sunday, Interior Minister Mohamed Ibrahim said.
The arrests were the biggest police sweep targeting Morsi supporters in a single day since Sisi came to power after a landslide election victory last May.
Ibrahim said 20 people were killed Sunday in clashes, most of them in Cairo's northern district of Matareya, adding two policemen were among the dead.
A health ministry official said among the dead was a protester killed in the northern city of Alexandria in similar clashes.
Late on Monday the interior ministry said it had deployed more police forces to Cairo's Matareya district where fresh skirmishes were reported.
Sunday's death toll from clashes was the biggest in a single day since Sisi came to office.
Three suspected militants also died when they mistakenly blew themselves up while planting explosives in the Nile Delta region.
-'EXCESSIVE FORCE' -
The authorities have blamed the Muslim Brotherhood for the violence that has rocked Egypt since Morsi's ouster, including bombings and shootings targeting security forces.
However, more than 1,400 people have been killed in a government crackdown against Morsi's supporters, while over 15,000 have been imprisoned since he was toppled.
Dozens have also been sentenced to death in trials which the United Nations says are "unprecedented in recent history".
The Brotherhood has denied government accusations of involvement in attacks on security forces, mostly claimed by jihadist groups.
Egypt's deadliest militant group, Ansar Beit al-Maqdis, said in a video issued Monday that it executed a policeman kidnapped in Rafah bordering the Gaza Strip earlier this month.
The United States, Britain and Human Rights Watch condemned Egypt's deadly use of force against protesters.
"Four years after Egypt's revolution, police are still killing protesters on a regular basis," said Sarah Leah Whitson, Middle East and North Africa director of HRW.
The New York-based HRW called for "an independent investigation into the authorities' excessive use of force" to quell "apparently peaceful protests."
Ibrahim dismissed the criticism.
"This organisation has never been objective in its reports," he said, blaming the Brotherhood for Sunday's violence.
US State Department spokeswoman Jen Psaki urged the "security forces to show restraint and to provide a safe environment in which Egyptians can peacefully express their views."
The British Foreign Office said it was "deeply concerned by the use of deadly force by the police against demonstrators".
Tensions had surged on the eve of the January 25 anniversary, when a female demonstrator was killed in clashes with police during a rare leftwing protest in Cairo.
Shaima al-Sabbagh died of birdshot wounds when police opened fire to disperse a march, fellow protesters and HRW said.
An 18-year-old female protester was also killed on Friday in clashes in Alexandria.
- MUBARAK SONS WALK FREE -
Sisi has been regularly accused by activists and rights groups of installing a regime that is more repressive than Mubarak's.
His supporters deny the allegations, pointing to his popularity among a large section of Egypt's population weary of four years of turmoil and economic crisis.
Ibrahim also said that Mubarak's sons Alaa and Gamal -- symbols of corruption during their father's rule -- had been released from jail pending a retrial in a graft case, four years after their arrest.
When asked by a reporter when they were actually released, Ibrahim said: "How does it concern you when they were released?"
Their release so close to the anniversary of the 2011 revolt presents a dilemma for Sisi, who is accused by opponents of reviving Mubarak-era practices.