Seven people were killed in an attack on a military post near a protest rally in Tehran against the re-election of President Mahmoud Ahmadinejad, who arrived in Russia on Tuesday to attend an international meeting, state media said Tuesday.
"Several thugs wanted to attack a military post and vandalize public property in the vicinity of Azadi Square," radio Payam reported, referring to the site of the rally held on Monday.
Both camps planned rallies for Tuesday, with a demonstration in support of Ahmadinejad set to take place at the same Tehran site where supporters of defeated presidential candidate Mir Hossein Mosuavi plan to gather just two hours later, the official IRNA news agency said.
The Iranian capital has already seen three days of the biggest and most violent anti-government protests since the 1979 Islamic revolution, and Mousavi supporters have pledged to continue their demonstrations even as their unsuccessful candidate appealed for calm at the planned rally.
Iran's English-language Press TV said seven people were killed and several wounded at the end of Monday's opposition rally -- a mainly peaceful gathering attended by many tens of thousands -- when "thugs" tried to attack a military post in central Tehran.
It gave no details of how the seven deaths occurred.
An Iranian photographer at the scene had said Islamic militiamen opened fire when people in the crowd attacked a post of the Basij religious militia.
The Iranian capital has already seen three days of the biggest and most violent anti-government protests since the 1979 Islamic revolution, A man was shot dead and many others wounded during a rally by hundreds of thousands of opposition supporters on Monday, a witness said.
"Tomorrow at 5 p.m. (1230 GMT) at Vali-ye Asr Square," some of the crowd chanted at Monday's march, referring to a road junction in the city of 12 million people.
Mousavi said Monday he was "ready to pay any price" in his fight against election irregularities, his Web site quoted him as saying, indicating a determination to keep up the pressure for the election result to be annulled.
Some formed a human chain in front of a building of the Basij Islamic militia but others broke through and paramilitaries opened fire on the crowds, sending thousands fleeing in havoc.
Britains' Channel 4 television showed footage of protesters surrounding the building and setting it ablaze. They hurled stones at militiamen on the rooftop, who fired into the air then into the crowd.
Members of Iran's security forces have at times fired into the air during the unrest and used batons to beat protesters who have pelted police with stones.
The Basij militia is a volunteer paramilitary force loyal to Supreme Leader Ayatollah Ali Khamenei who has the final say on all matters of state in Iran.
Gunfire was also heard in three districts of wealthy northern Tehran on Monday evening and residents said there had also been peaceful pro-Mousavi demonstrations in the cities of Rasht, Orumiyeh, Zahedan, and Tabriz on Monday.
Further protests, especially if they are maintained on the same scale, would be a direct challenge to authorities who have kept a tight grip on dissent since the overthrow of the U.S.-backed shah after months of demonstrations 30 years ago.
Despite the protests, Ahmadinejad flew to Russia for a meeting of the Shanghai Cooperation Organization of Central Asian powers.
U.S. President Barack Obama said on Monday he was "deeply troubled" by the violence in Iran.
"The democratic process, free speech, the ability of people to peacefully dissent -- all those are universal values and need to be respected," he said, but added that the U.S. respects Iranian sovereignty.
The United States and its European allies have been trying to engage Iran and induce the world's fifth-biggest oil exporter to halt nuclear work that could be used to make an atomic bomb. Iran says it only wants nuclear energy to generate electricity.
Obama said he would continue pursuing tough, direct dialogue with Tehran but urged that any Iranian investigation of election irregularities be conducted without bloodshed.
Demonstrators filled a broad avenue in central Tehran for several kilometers (miles) on Monday, chanting "We fight, we die, we will not accept this vote rigging," in support of Mir Hossein Mousavi, the defeated moderate candidate.