The announcement came a few hours before the United Nations denounced what it called “excessive force” by authorities on the border between Morocco and Spain and demanded an investigation into the migrants’ deaths.

The tragedy happened at dawn on Friday when around 2,000 migrants, mostly from sub-Saharan Africa, tried to break through the fence from Morocco into the tiny Spanish enclave.

Moroccan authorities said some had fallen while trying to scramble over the fence, giving an initial toll of 18 dead, but later raising it to 23 after another five migrants died of their injuries.

They said 140 Moroccan police were wounded.

Very few details about the incident were available, but Spanish media showed images of many people lying on the ground, some with bloodied hands and torn clothes.

“We want to know what happened so we can explain it to the relatives of those who died,” said Ahmed, an Eritrean migrant who described Friday’s incident as “a massacre”.

He was among around 50 migrants who held a protest on Tuesday in front of the UN refugee agency (UNHCR) headquarters in the Moroccan capital Rabat, some raising signs reading “stop killing us”.

“They beat us inhumanely,” said Omar, a migrant who said he was fleeing “war and prison” at home in Sudan.

“We don’t feel safe here, our lives are in danger,” he told AFP.

PASCOMS, an association for sub-Saharan migrants in Morocco, blamed the European Union, its member states and Morocco for what it called a “disaster”.

‘Migrants beaten with batons’

The death toll was by far the worst recorded in years of attempts by migrants to cross into Melilla, one of Spain’s two North African enclaves which have the EU’s only land borders with Africa, making them a magnet for migrants desperate to escape grinding poverty and hunger.

READ ALSO: Why are Ceuta and Melilla Spanish?

In a statement, the Spanish prosecutors’ office said the decision was made by Attorney General Dolores Delgado in order “to clarify what happened at the Melilla border”, citing the “seriousness and gravity” of the incident.

UN Secretary-General Antonio Guterres’s spokesman, Stephane Dujarric, told reporters in New York that the “use of excessive force by the authorities” was “unacceptable” an should be investigated.

Earlier on Tuesday, the United Nations’ rights office had called for an independent investigation “as a first step towards establishing the circumstances of the deaths and injuries”, spokeswoman Ravina Shamdasani said.

While it remained unclear how they had died, Shamdasani said the office had received reports of “migrants beaten with batons, kicked, shoved, and attacked with stones by Moroccan officials as they tried to scale the barbed-wire fence” which is between six and 10 metres high.

Meanwhile Morocco, locked in a worse-than-usual standoff with neighbouring Algeria, blamed its regional rival for “deliberately lax” control of their shared border, according to a statement from its Madrid embassy carried by Spanish media.

Algerian diplomat Amar Belani, charged with the Western Sahara dossier that is at the heart of tensions between Rabat and Algiers, said Morocco was looking for “scapegoats to relieve itself of its responsibilities”.

The African Union has also called for an “immediate investigation”, with AU Commission chief Moussa Faki Mahamat expressing “deep shock and concern at the violent and degrading treatment” of migrants at the border.

In Morocco, prosecutors are moving to press charges against 65 migrants, mostly Sudanese, who tried to storm the border, a defence lawyer in Rabat said on Monday.

READ MORE: What happens to undocumented migrants after they arrive in Spain?