Human rights activists are seeking an independent inquiry after an Egyptian police operation to break up a protest camp left more than 20 Sudanese dead.
Thousands of riot police took part in the dawn operation in Cairo
"The high loss of life suggests the police acted with extreme brutality," said New York-based Human Rights Watch.
UN Secretary General Kofi Annan said the deaths were a "terrible tragedy that cannot be justified".
Thousands of police wielding truncheons and firing water cannon at protesters stormed the Cairo camp early on Friday.
The Sudanese migrants had been camped outside UN offices since September, demanding that the UN refugee agency (UNHCR) move them to a third country with better conditions.
Latest reports in Cairo put the number of dead at 25; several are said to be children. The Egyptian authorities said 74 police were injured.
The interior ministry said there was a stampede that left protesters dead and injured. It also accused migrant leaders of inciting attacks against the police.
'Rush to judgement'
A Human Rights Watch statement said Egyptian President Hosni Mubarak should "urgently" appoint an independent commission to investigate the use of force.
"A police force acting responsibly would never have allowed such a tragedy to occur," HRW deputy director Joe Stork said.
The UN High Commissioner for Refugees, Antonio Guterres, said there was no justification for the violence and loss of life.
However Egypt has criticised Mr Guterres for making "hasty" judgements without being in full possession of the facts.
It expressed its sorrow over the incident, but said the UN had asked the police to intervene to end a long-running protest outside its offices.
UN spokeswoman Astrid van Genderen Stort said the agency had repeatedly urged the authorities to resolve the protest peacefully.
The Sudanese migrants were forced onto buses
She also said the UN had so far been denied permission to visit the migrants.
A Khartoum news paper called the deaths "unacceptable" and urged Sudan to take legal action against the Egyptian police "for provoking violence."
"This is a very sorrowful end for a people who fled their own country's frying pan only to fall into Egypt's fire. The inappropriate use of brutal force was uncalled for and appallingly inhuman," the Khartoum Monitor said.
Witnesses said some refugees stood defiantly or fought back, while others fled after police stormed the ramshackle encampment at 0500 (0300GMT) on Friday.
They said the migrants, including women and small children, were dragged towards buses as they tried to resist, leaving clothes, suitcases and makeshift tents scattered in their wake.
One Sudanese asylum-seeker, Napoleon Roberts, told the BBC he had been taken to a barracks south of the capital and was being held with about 1,700 others in disgusting conditions.