Iranians planned more protests on Tuesday against a presidential election they say was rigged but the focus of their anger, President Mahmoud Ahmadinejad, left the country to attend an international meeting.

The Iranian capital has already seen three days of the biggest and most violent anti-government protests since the 1979 Islamic revolution after the hardline Ahmadinejad was declared winner of last Friday's vote.

Islamic militiamen killed a man during a march by tens of thousands of opposition supporters on Monday, a witness said.

Tomorrow at 5 p.m. (8:30 a.m. EDT) 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.

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 on Tuesday 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.

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.

WE FIGHT, WE DIE

Demonstrators filled a broad avenue in central Tehran for several km (miles) on Monday, chanting We fight, we die, we will not accept this vote rigging, in support of Mirhossein Mousavi, the defeated moderate candidate.

Mousavi said 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.

One man was killed and many wounded, said an Iranian photographer who witnessed the shooting. Television footage showed one man, his leg covered with blood, being bundled onto the back of a taxi and driven away.

Britain's 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.

Tanks and guns have no use any longer, chanted the protesters in a deliberate echo of slogans used leading up to the 1979 revolution.

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.

(Editing by Jon Hemming and Angus MacSwan)

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