The government moved to shore up its position a day after its security forces killed at least 15 protesters and wounded scores more in the worst clashes since the disputed presidential election in June.
The opposition said 1500 people had been arrested nationwide since the violence flared, including 1110 in Tehran and 400 in the central city of Isfahan.
The brutality on the Shia holiday of Ashura was condemned around the world.
The US administration cast aside its reticence and denounced the crackdown.
US President Barack Obama condemned the Tehran regime's "iron fist of brutality" and the "violent and unjust suppression of innocent Iranian citizens".
During the turmoil after Iran's controversial presidential elections in June, Mr Obama - then in the midst of a diplomatic outreach to Iran - was criticised for his relative silence on the protests.
France deplored the "arbitrary arrests and violence carried out against ordinary protester". Germany, Italy, Austria, Britain, Canada and Norway issued similar statements. Even Russia, one of Iran's main trading partners, called for restraint.
Those killed included the opposition leader's nephew, Seyed Ali Mousavi, who was shot in the chest. Tehran was rife with speculation that he had been assassinated to send a message to his uncle, and the government moved rapidly to prevent his death becoming another rallying point for the opposition.
Security forces ringed the hospital where his body was taken and used teargas to disperse protesters outside. It later emerged they had removed his body and taken it to an undisclosed location.
"My brother's body was taken away from the hospital and we cannot find it," Seyed Reza Mousavi, his brother, told the reformist website Parlemannews. "Nobody accepts responsibility for taking away the body . . . We cannot have a funeral before we find the body."
Iranian authorities said they were holding the bodies of five slain anti-government protesters, including the nephew of the opposition leader, in what appeared to be an attempt to prevent activists from using their funerals as a platform for more demonstrations.
The violence came just days before a year-end deadline imposed by the Obama administration for Tehran to show good faith in its negotiations with the West over its nuclear program, or face new sanctions. Senior US officials said the Obama administration recogniseds that it was now at a "pivot point" in its strategy on Iran.
Tehran's crackdown on protests may be fanning fresh anger at the regime in Iran.
But a significant weakening of public support for President Mahmoud Ahmadinejad could be a mixed blessing for US and Western officials now trying to determine how the domestic unrest could affect negotiations with Iran.
A weakened regime could be less inclined to make concessions over its nuclear program, a source of pride for many Iranians, they say.
Sanctions, meanwhile, could help shore up Mr Ahmadinejad's support by whipping up nationalistic anger at foreign meddling.