The EU said Tuesday that it deplored the deaths of seven demonstrators in Iran following its disputed presidential election.
EU spokesman Amadeu Altafaj Tardio quoted the European Commission as being "extremely worried" over the violent protests in Teheran and other Iranian cities.
Altafaj Tardio said it was "a deplorable thing" that seven people died in clashes reported on Iranian state radio. He reiterated calls for the right of Iranian citizens to demonstrate peacefully.
Allegations of vote rigging and election fraud have led to thousands of people joining rallies supporting reformist candidate Mir Hossein Mousavi, who claims last week's ballot was fixed to re-elect hardline Iranian President Mahmoud Ahmadinejad.
Up to 24 people were reportedly killed and many more were injured in clashes with pro-government forces Monday night, according to non-Iranian news services worldwide Tuesday.
State TV announced Tuesday midday that the "main agents" behind the unrest have been arrested and searches have turned up weapons and explosives.
The EU joined the US in expressing growing international concern over the current situation in Iran.
On Monday, President Barack Obama said the world was inspired by the outpouring of Iranian political dissent and an inquiry into the disputed presidential election should go ahead without violence.
Obama said that he does not know who rightfully won the Iranian election, but that Iranians have a right to feel their ballots mattered.
His response marked the most extensive US response to Friday's voting, and appeared calculated to acknowledge the outpouring of dissent in Iran without claiming any credit.
"It would be wrong for me to be silent on what we've seen on the television the last few days," Obama told reporters at the White House.
He added, however, that "sometimes, the United States can be a handy political football."
The American president is personally hugely popular in Iran, and all candidates in this year's surprisingly lively Iranian presidential election trod carefully on criticism of the United States as a result. But the larger idea of the United States, and its world influence backed by massive military power, remains highly divisive. Any candidate or popular movement seen to have the express backing of the United States would probably be doomed.
"What I would say to those people who put so much hope and energy and optimism into the political process, I would say to them that the world is watching and inspired by their participation, regardless of what the ultimate outcome of the election was," Obama said. "And they should know that the world is watching."
The US on Monday expressed concern about continued turmoil in Iran over election irregularities but refused to characterize the process as fraudulent.
"They are deeply troubling," said State Department spokesman Ian Kelly about allegations of fraud. "We are in a position of still assessing what went on. And it's difficult to assess because there weren't any international monitors at the elections."
White House Spokesman Robert Gibbs also described "concern about what we've seen," but at the same time he added, "We continue to be heartened by the enthusiasm of young people in Iran."
Neither indicated that the US government would change its approach to negotiations with Teheran over its nuclear program as a result of the elections and the allegations that have accompanied them.