By Jay Deshmukh

TEHRAN, Jun 19, 2009 (AFP) - In a rare move, Iran's supreme leader Ayatollah Ali Khamenei was due to lead weekly Muslim prayers here Friday after tens of thousands of supporters of defeated presidential candidate Mir Hossein Mousavi demonstrated for a sixth straight day.

Keeping up the pressure on the Islamic regime over the disputed vote, the rally came as the electoral watchdog, the Guardians Council, said it was investigating 646 complaints of poll violations from Mousavi and the other two losing candidates.

The council also invited the trio to set out their grievances on Saturday, with a decision on Sunday about any possible recount in the June 12 election which returned hardline incumbent Mahmoud Ahmadinejad to power.

On Thursday Mousavi joined the crowds, most of whom were dressed in black as a mark of mourning for demonstrators killed in clashes during the protests, which have been banned by the authorities, witnesses said.

The crowds dispersed quietly about five hours later, witnesses said.

State radio has reported seven deaths since the protests started.

Foreign journalists have been barred from attending all rallies or other events without express authority.

Protesters carried pictures of Mousavi -- who is demanding a new election -- and placards bearing slogans such as "We have not had people killed to compromise and accept a doctored ballot box," one witness said.

Mousavi appeared despite warnings from the hardline Basij militia -- which has been at the forefront of action against protests -- for defeated candidates to dissociate themselves from "rioters."

Facing their biggest crisis since the 1979 revolution against the shah, Iran's Islamic rulers have gone on the offensive.

They arrested protesters and prominent reformists, tightened their grip on the media and lashed out at "meddling" by foreign foes, including the United States.

Khamenei, who will lead Friday's prayers in the presence of the Basij militia, said he would consider a partial recount after Ahmadinejad's defeated challengers lodged formal complaints of vote-rigging.

Meanwhile state television reported that a plot with "extensive ties" to unnamed foreign interests had been uncovered to carry out bombings in Tehran on election day.

World powers have raised concern about the violence and widespread arrests, with EU leaders set Friday to condemn their use against protesters.

According to a draft statement Tehran will also be urged to investigate the claims of electoral fraud.

"The European Union is observing the response to the protests across Iran with serious concern," said the statement to be agreed by the European leaders at a two-day Brussels summit.

"It strongly condemns the use of violence against protesters resulting in the deaths of several people," continues the text, seen by AFP.

On Wednesday, Iran had summoned the Swiss ambassador, who represents US interests in Iran, to protest at what it called "interfering remarks" by American officials.

The White House Thursday defended President Barack Obama's stance on Iran's crisis, amid fresh Republican demands for him to strongly back demonstrators.

"The president believes that he's struck the right tone as do others in the administration, as do others in the Republican Party, as do others in the Democratic Party," said White House spokesman Robert Gibbs.

Obama has warned that universal rights of peaceful protest should be honored, but has refused to pick sides in the post-election showdown.

Canadian Foreign Minister Lawrence Cannon has rejected Tehran's urging that Ottawa "stay out" of Iran's internal politics.

In a sign of cracks emerging within the Iranian elite, several influential clerics spoke out about the election results and the subsequent crackdowns.

Ayatollah Mehdi Hadavi Tehrani called on Thursday for Interior Minister Sadeq Mahouli to be impeached.

The combative Ahmadinejad -- who set Iran on a collision course with the West during his first four-year term -- remained defiant, saying his victory showed faith in his government of "honesty and service to the people".

In new measures against the media, the elite Revolutionary Guards told websites to delete material that "creates tensions" or face legal action, while the foreign ministry accused some media of becoming the "mouthpiece of the rioters' movement."

The media curbs prompted protests Thursday in Paris, where several dozen journalists and the press watchdog Reporters Without Borders staged a rally outside the Iranian embassy.

Pictures, videos and updates from Iran's streets continue to pour in to social-networking and image-sharing websites such as Twitter, Facebook and YouTube despite Iranian efforts to cut off mobile phones and the Internet.

Some jumpy amateur video footage broadcast on the Internet has shown chaotic and sometimes brutal scenes of violence, with police beating protesters. One image purportedly showed a protester shot dead during Monday's massive demonstrations.

Human rights groups on Thursday urged US lawmakers to revive rules that would punish US firms which help authoritarian governments crack down on local computer users.

burs/gk/ch

Copyright AFP 2009.