Cairo (AFP) ― 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.