The World Organisation Against Torture (OMCT) and the vice president of a UN human rights body said they feared that at least 35 people arrested during a security operation in a camp for Iranian exiles in Iraq could be sent back to Iran and near certain death.
Eleven people were killed in the raid, Iraqi authorities said, but Iranian National Council for Resistance members said here that 13 people had died and 36 were arrested.
Eric Sottas, secretary general of the OMCT, said there had been multiple warnings about a possible clampdown on members of the People's Mujahedeen group and their families in the Ashraf camp as Iraq and Iran forged closer ties.
He said the international community had demonstrated an "incomprehensible sort of passivity" and called for an investigation into the incident.
"There's no way that the protection offered by the Iraqis would have been sufficient bearing in mind the pressure that has been exerted," Sottas told reporters.
Those who had been arrested faced "summary executions" if they were transferred to Iran, he said.
"The most practical thing would be that the Americans assume their responsibility and intervene again to ensure protection (and) that should later be transferred to an international force", Sottas said.
About 3,500 Mujahedeen and their families have lived in Ashraf since former Iraqi dictator Saddam Hussein allowed the Iranian opposition to set up bases on his territory during his 1980-88 war with Iran.
Following the 2003 US-led invasion of Iraq, American forces disarmed the Mujahedeen in Ashraf but placed the residents under protection.
Iraq's increasingly independent government has moved to take charge of the site.
"It's shameful. The United States and the European Union should be ashamed of what they did," said Jean Ziegler, vice president of the consultative committee of the UN Human Rights Council.
Elaheh Azinefaer of the Iranian exile National Council for Resistance also accused the International Committee of the Red Cross (ICRC) of standing by despite the warnings.
ICRC spokesman Florian Westphal told AFP that the humanitarian agency, which only gained access to Ashraf in 2008, had been following the situation "closely" and was in contact with Iraqi authorities.
"It is the responsibility of the concerned authorities to treat these people in the limits of the law," he added.
