More than a thousand Sudanese rallied Thursday decrying sexual attacks, after the UN said at least 13 women and girls were raped during recent mass protests against the army.

"Rape will not stop us", the crowd chanted at protests in the capital Khartoum and its twin city Omdurman.

"The oppression that we're subjected to... through the crime of rape is a systematic tactic to crush our spirit," protester Malaz Kamal told AFP.

"We're telling them (the perpetrators) whatever you do, we will still come out and make our voices heard," the 25-year-old said.

Fellow demonstrator Nahla Issa, 23, said: "We want to restore the rights of raped women, not only in Khartoum recently... This can only be done by uprooting this current coup regime".

The European Union and the United States issued a joint statement Thursday condemning the use of sexual violence "as a weapon to drive women away from demonstrations and silence their voices".

The UN said it had "received allegations that 13 women and girls were victims of rape or gang rape" during protests on Sunday, UN High Commissioner for Human Rights spokesperson Liz Throssell said on Monday.

Security forces fired tear gas and live bullets Sunday as they attempted to disperse hundreds of thousands of marchers protesting against the October 25 coup led by military chief General Abdel Fattah al-Burhan.

Sunday marked the third anniversary of mass demonstrations that led to the ouster of veteran strongman Omar al-Bashir.

The UN said it had also "received allegations of sexual harassment by security forces against women who were trying to flee" the area around the presidential palace in Khartoum on Sunday evening.

Two people died during Sunday's march, taking the death toll in protest-related violence to at least 47 since the military takeover, according to the independent Doctors' Committee.

The generals placed civilian Prime Minister Abdalla Hamdok under effective house arrest after the coup -- but reinstated him on November 21 under a deal promising elections for July 2023.

But the move alienated many of Hamdok's pro-democracy supporters, who dismissed it as a gift to Burhan that provided a cloak of legitimacy for his takeover.

Yasir Arman, a former adviser to Hamdok and a leading figure in the pro-democracy Forces for Freedom and Change (FFC) coalition, told a press conference Thursday that there needed to be a "transparent investigation... and the perpetrators must be held accountable".

The FFC -- an umbrella group which spearheaded the protests against Bashir -- has urged people to continue to reject military rule, with further demonstrations planned for December 25 and 30.