A top aide to Iran's supreme leader called the country's main opposition figure a U.S. agent and said in an editorial Saturday he should be tried for committing crimes against the nation.

While hard-line figures had previously demanded Mir Hossein Mousavi to be prosecuted for describing Iran's June 12 elections fraudulent and leading demonstrations afterward, the editorial was the first public declaration that the opposition leader was a foreign agent.

After quashing the post-election street demonstrations, Iran's leadership has been trying to erase any lingering doubts about the legitimacy of President Mahmoud Ahmadinejad's re-election by portraying the unrest as sparked by foreign meddling.

"It has to be asked whether the actions of (Mousavi and his supporters) are in response to instructions of American authorities," said Hossein Shariatmadari in an editorial appearing in the conservative daily Kayhan.

Shariatmadari doesn't hold a government position but is the powerful director of the Kayhan newspaper group and a close adviser to Iran's supreme leader Ayatollah Ali Khamenei. He added that Mousavi was trying to "escape punishment for murdering innocent people, holding riots, cooperating with foreigners and acting as America's fifth column inside the country."

He called for Mousavi and former reformist president Mohammad Khatami to be tried in court for "horrible crimes and treason," adding that there were "undeniable documents" proving Mousavi's foreign links.

On Friday, another powerful hardline cleric said that Iran would put detained local employees of the British embassy on trial for being involved in the demonstrations, in a further effort to prove foreign elements were behind the unrest.

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The announcement fueled calls in Europe for tougher action against Tehran. Britain is pressing for members of the European Union to pull their ambassadors out of Tehran to protest the staffers' arrests last week.

Police say 20 "rioters" were killed during the violence as well as eight members of the paramilitary Basij militia tasked with putting down the protests. More than a thousand people have been arrested.

There have been no street protests since Sunday, but Mousavi has maintained his opposition to the results, issuing a defiant statement on Wednesday that he considered the government illegitimate and demanded political prisoners be released.

"A majority of the people — including me — do not accept its political legitimacy," Mousavi said. "There's a danger ahead. A ruling system which relied on people's trust for 30 years cannot replace this trust with security forces overnight."

Mousavi has been laying low, however, and made no public appearances after the Basij on Wednesday formally requested that he be investigated for the protests.

Iran's ruling clerics have called the elections "pure" and "healthy" following the supreme leader's declaration that the results would stand.

Only one of top clerics in the religious center of Qom have congratulated Ahmadinejad's re-election, showing their displeasure with the disputed results.

Some have even openly supported Mousavi and condemned the government's tactics against demonstrators and expressed their own doubts about the election results.

"A large portion of the people have not been convinced over the ambiguities in the election ... Due to lack of public support, the government may faces legal and civil problems and a lack of competency," Grand Ayatollah Youssef Saanei said in a statement on his Web site late Friday.

Saanei is one of the top nine most influential clerics in the country and has substantial following among Iranians, though he is on poor terms with the government.

"I remind all forces that are required to protect the ... people and that no order should be an excuse or permission to violate the rights of the people ... let alone killing or injuring them," his statement added.