Reformist leader Mir Hossein Mousavi says the election that put Ahmadinejad back in office was fraudulent and that the regime is "illegitimate."
He called for Mousavi and former reformist president Mohammad Khatami to be tried in court for "horrible crimes and treason."
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.
Shariatmadari, who holds no official position but is a close adviser to Iran's supreme leader Ayatollah Ali Khamenei, wrote 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."
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 another apparent effort to prove foreign elements were behind the unrest.
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.
A lawyer for one of the charged staffers -- embassy chief political analyst Hossein Rassam -- said his client has been accused of "acting against national security."
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. Part of the government's strategy appears to be extracting "confessions" from reformers under duress or torture to prove alleged involvement in a "velvet revolution" conspiracy, The New York Times reported.
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.