Iran braced for a fourth day of massive protests Thursday by opponents of President Mahmoud Ahmadinejad in open defiance of the country's supreme leader, who has urged the nation to unite behind the Islamic state.

Opposition leader Mir Hossein Mousavi urged supporters to wear black Thursday to the planned rally in mourning for the alleged election fraud during Friday's vote and lives lost during this week's protests.

Seven demonstrators were shot Monday by pro-regime militia in the first confirmed deaths since the unrest erupted after the election, which the government said Ahmadinejad won in a landslide victory.

Thursday's protest would be the fourth straight day of major marches in Teheran, including hundreds of thousands of people Monday in a huge procession that recalled the protests of the Islamic Revolution. Mousavi's Web site said he may join the rally on Thursday to be held at a downtown Teheran square. RELATEDUS shrugs off Iran 'meddling' claimsIranian soccer team wears green bandsFarkash: Street protests the beginning of the end for Iran

Authorities have rounded up perceived dissidents and tried to further muzzle Web sites and other networks used by Mousavi's backers to share information and send out details of Iran's crisis after foreign journalists were banned from reporting in the streets.

On Thursday, Mousavi's Web site said that both Mousavi and former reformist President Mohammad Khatami sent a joint letter to Iran's head of judiciary, Ayatollah Mahmoud Hashemi Shahroudi, asking him to take measures to stop violence against protesters by police and help to release detained demonstrators.

The Iranian government has directly accused the United States of meddling in the deepening crisis.

A statement by state-run Press TV blamed Washington for "intolerable" interference. The report, on Press TV, cited no evidence.

President Barack Obama said he shared the world's "deep concerns" but it was "not productive, given the history of US-Iranian relations, to be seen as meddling." The two countries severed diplomatic relations after the 1979 Islamic Revolution in the bloody showdown over allegations of vote-rigging and fraud.

The street protests have presented one of the gravest threats to Iran's complex blend of democracy and religious authority since the system emerged from revolution.

Any serious shift of the protest anger toward Iran's non-elected theocracy would sharply change the stakes. Instead of a clash over the June 12 election results, it would become a showdown over the core premise of Iran's system of rule - the almost unlimited authority of the clerics at the top.

For the moment, however, both sides appear to be using the same tactics since the disputed results showed hard-line Ahmadinejad as the landslide winner.

A crackdown on dissent continued, with more arrests of opposition figures reported, and the country's most powerful military force - the Revolutionary Guard - saying that Iranian Web sites and bloggers must remove materials that "create tension" or face legal action.

The government has blocked certain Web sites, such as BBC Farsi, Facebook, Twitter and several pro-Mousavi sites that are vital conduits for Iranians to tell the world about protests and violence. Many other sites, including Gmail and Yahoo, were unusually slow and rarely connect.

Mousavi has condemned the blocking of Web sites, saying the government did not tolerate the voice of the opposition.

Iran's main electoral authority has said it was prepared to conduct a limited recount of ballots at sites where candidates claim irregularities. The re-count would be overseen by the Guardian Council, an unelected body of 12 clerics and Islamic law experts close to Supreme Leader Ayatollah Ali Khamenei.

Mousavi alleges the Guardian Council is not neutral and has already indicated it supports Ahmadinejad. He wants an independent investigation.