A number of opposition figures have been arrested in Iran, a day after at least eight people died during the most violent protests for months.
Those detained include senior aides to opposition leader Mir Hossein Mousavi, and a former foreign minister.
Mr Mousavi's nephew was among those killed on Sunday. Officials deny opposition claims police shot them.
US President Barack Obama has condemned the crackdown and urged Iran to release those "unjustly detained" immediately.
State media forensic tests were being carried out on the body of Mr Mousavi's nephew, Seyed Ali Mousavi, and the four other people killed on Sunday, preventing the rapid burials that are usual under Islamic tradition.
The bodies had been "retained in order to complete forensic and police examinations and find more leads on this suspicious incident", the Irna news agency reported.
RECENT UNREST IN IRAN
19 Dec: Influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri dies aged 87
21 Dec: Tens of thousands attend his funeral in Qom; reports of clashes between opposition supporters and security forces
22 Dec: Further confrontations reported in Qom
23 Dec: More clashes reported in city of Isfahan as memorial is held
24 Dec: Iran reportedly bans further memorial services for Montazeri except in his birthplace and Qom
26 Dec: Clashes reported in central and northern Tehran
27 Dec: At least eight dead following anti-government protests in Tehran; 300 reported arrested
The Mousavi family earlier said Seyed Ali's body had been taken without their permission from the hospital where it was being held.
Opposition sources said the body had been taken by government agents in order to prevent his funeral becoming a rallying point for more protests.
An opposition website, Norooz, said police had fired tear gas on Monday to disperse a group of Mousavi supporters who were demonstrating outside the hospital.
According to Mr Mousavi's website, Seyed Ali Mousavi was shot in the back on Sunday as security forces fired on demonstrators in Tehran.
Intermittent protests in Iran following President Mahmoud Ahmadinejad's controversial re-election in June have represented the biggest challenge to the government since the 1979 Islamic revolution.
Foreign media face severe restrictions in Iran, making reports hard to verify.
BBC Tehran correspondent Jon Leyne, reporting from London, says the government's immediate response to the latest confrontation has been to arrest senior opposition figures, as it did after protests against the disputed presidential elections in June.
The authorities are blaming troublemakers for the violence, our correspondent says, with the police suggesting that protesters may have shot each other.
The majority hardline block in the Iranian parliament called on "security and judiciary authorities to firmly deal with those who mock Ashura", referring to the Shia Muslim festival that reached its climax on Sunday.
But members of the opposition believe Seyed Ali Mousavi was targeted by the government in an attempt to intimidate Mir Hossein Mousavi.
Our correspondent adds that the government will be doing itself no favours if it has taken his body because this would outrage religious conservatives, as well as the opposition.
Among those reported arrested on Monday were opposition politician Ebrahim Yazdi, a foreign minister after the 1979 revolution and now leader of the Freedom Movement of Iran, his nephew, Lily Tavasoli.
Ebrahim Yazdi, pictured in 2005, was also arrested in June this year
Mr Yazdi's son Khalil, who lives in the US, told the BBC's World Today programme he believed the Iranian authorities wanted to close down all opposition groups.
"It is a shameless and irresponsible act," he said.
"Any opposition now, they want to shut [it] down. We're going down a one-way street that's now going downhill."
The Parlemannews website reported that three aides to Mir Hossein Mousavi had been arrested.
It also named two aides to reformist former President Mohammad Khatami as being among those rounded up by the authorities.
Mousavi Tebrizi, a senior cleric from the holy city of Qom who is close to Mr Mousavi, is also reported to have been arrested, as is human-rights campaigner and journalist Emeddin Baghi.
On Monday, state-owned English-language Press TV said eight people had died. Earlier, Persian state television had reported at least 15 people killed.
The official death toll for Sunday's confrontation is the highest since June, and police said about 300 people had been detained.
Unconfirmed reports, later denied by a local prosecutor, said four people also died in protests in the north-western city of Tabriz. Clashes were also reported in Isfahan and Najafabad in central Iran and Shiraz in the south.
Moderate cleric Mehdi Karoubi, who came fourth in last June's election, criticised Iran's rulers for Sunday's violence, an opposition website reported.
The US, the UK, France, Germany and Canada have all condemned the violence.
Mr Obama said the protests were not about the US but the "Iranian people and their aspirations for justice and a better life for themselves".
"The decision of Iran's leaders to govern through fear and tyranny will not continue," he said.
British Foreign Secretary David Miliband said it was "particularly disturbing to hear accounts of the lack of restraint by the security forces" on a day of religious commemoration.
German Chancellor Angela Merkel criticised the "unacceptable actions of the security forces" and urged Tehran to respect civil rights.
Iranian security forces have been on alert since influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri died a week ago aged 87.
His funeral attracted tens of thousands of pro-reform supporters, many of whom shouted anti-government slogans.
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