Foreign Secretary David Miliband praised the "courage" of protesters in Iran today after bloody clashes that are thought to have left more than a dozen dead.
Mr Miliband condemned the lack of restraint by Tehran's security forces, saying that reports emerging from the capital were "disturbing".
"The tragic deaths of protesters in Iran are yet another reminder of how the Iranian regime deals with protest," he said in a statement.
"Ashura is a time of religious commemoration and reflection and it is therefore particularly disturbing to hear accounts of the lack of restraint by the security forces.
"Ordinary Iranian citizens are determined to exercise their right to have their voices heard. They are showing great courage.
"I call on the Iranian Government to respect the human rights of its own citizens - rights which Iran has promised to respect."
A number of opposition figures are believed to have been detained in Iran today, as the violence abated.
Those held apparently include senior aides to opposition leader Mir Hossein Mousavi, whose nephew Seyed Ali Mousavi was among those killed in yesterday's clashes - the worst since June's contested elections.
State television has given different estimates of the casualties, ranging from eight to 15 dead. Most of those are thought to have been protesters, although several members of the security forces were also reportedly killed.