As tens of thousands of supporters of reformist leader Mir Hossein Mousavi gathered in Tehran Wednesday, Iran accused the United States of "intolerable" meddling in its internal affairs.
The White House dismissed the allegation.
President Barack Obama has been clear that there is "a vigorous debate in Iran, between Iranians, about their leadership," spokesman Robert Gibbs said, adding Obama stands by his defence of democratic principles including the right of people to demonstrate peacefully.
Mousavi has accused Iran's government of rigging the election in favour of hardline President Mahmoud Ahmadinejad.
The two countries broke off diplomatic relations after the 1979 Islamic Revolution. The government summoned the Swiss ambassador, who represents U.S. interests in Iran, to complain about American interference, according to an English-language state-run channel.
Meanwhile, Mousavi's supporters gathered in Tehran's central Haft-e Tir Square for a silent protest, even as authorities warned citizens against promoting the rallies online.
Most of the demonstrators dressed in black with green wristbands and headbands as they quietly flashed victory signs with their hands, witnesses said.
The gathering formed as Mousavi posted a message online calling for another mass rally on Thursday.
The message on Mousavi's official website is seen as a direct challenge to Supreme Leader Ayatollah Ali Khamenei, who said Tuesday night that the opposition leader should pursue his demands through the country's electoral system. Khamenei also called on Iranians to unite behind their Islamic government as demonstrations continued over Friday's disputed presidential election results.
'Deviant news sites'
After Khamenei's appeal, the Revolutionary Guard, an elite military force answering to Khamenei, issued a statement on the state news service on Wednesday ordering any material that "creates tension" and encourages public disturbance and street riots be removed from Iranian websites and blogs.
Sites and bloggers who do not heed the warning will face legal action, the Revolutionary Guard said.
The statement also alleged the "deviant news sites" were backed by Canadian, American and British interests.
Iranian websites, as well as blogs, Facebook and Twitter, have been vital conduits for Iranians to inform the world about protests over the apparent landslide victory for Ahmadinejad.
All three of Ahmadinejad's challengers in Friday's election have alleged fraud after results showed the president winning by a 2-to-1 margin.
Protesters to gather at mosques
Iranian authorities have also restricted all journalists working for foreign media from first-hand reporting on the streets in an attempt to block images and eyewitness accounts from the rallies.
Mousavi's online message condemned the government's attempt to block the websites, saying the government did not tolerate the voice of the opposition.
"We are after a peaceful rally to protest the unhealthy trend of the elections and realize our goal of annulling the election results," Mousavi said.
Demonstrators should gather in mosques on Thursday to express solidarity with the families of the people killed in post-election unrest, the online statement said, urging his supporters to remain peaceful.
Mousavi said he would take part in Thursday's rally, but did not give details on when and where.
Supporters on both sides have taken to the streets in demonstrations and escalating violence that has seen at least seven people killed since the election.
The Guardian Council announced on Tuesday that it will conduct a partial recount of ballots at voting sites where candidates claim irregularities occurred.
According to government officials, Ahmadinejad won 62.6 per cent of the vote, while Mousavi garnered 33.75 per cent in a contest that appeared to divide urban and rural voters.
Mousavi has called the election an "astonishing charade" and demanded again on Tuesday its results be annulled and the vote be held again.
His representative, reformist cleric Ali Akbar Mohtashamipour, reiterated that demand after a meeting of the Guardian Council, an unelected body of 12 clerics and Islamic law experts close to the supreme leader and seen as supportive of Ahmadinejad.
"If the whole people become aware, avoid violent measures and continue their civil confrontation with that, they will win. No power can stand up to people's will," Mohtashamipour said. "I do not think that the Guardian Council will have the courage to stand against people."