Several thousand Russian ultra-nationalists whose ranks included fierce Kremlin critics marched through central Moscow on Sunday calling for President Vladimir Putin's resignation and an end to illegal immigration.

Armed with anti-Putin slogans and black and yellow flags of pre-revolutionary Russia, the black-clad participants in the "Russian March" took to the streets as Putin is facing the most vocal opposition to his rule since he came to power 12 years ago.

Russia on Friday blasted U.S. Secretary of State Hillary Clinton's call for an overhaul of the Syrian opposition and accused Washington of trying to solve the conflict on its own terms.

The Russian foreign ministry said Clinton's comments on the Syrian National Council (SNC) no longer representing the entire anti-regime movement clashed with the agreements world powers reached on the conflict in Geneva in June.

Russia said Thursday it was "realistic" to hold direct talks next month between Iran's chief negotiator and six-nation representative Catherine Ashton over the Islamic republic's nuclear drive.

"It would be realistic to talk about organizing one in November," Interfax quoted Deputy Foreign Minister Sergei Ryabkov as saying.

Thousands marched through Moscow on Saturday to protest against the rule of Vladimir Putin in a test of the opposition's challenge to the Russian president four months after his inauguration.

Waving nationalist flags, brandishing placards calling for early elections or wearing T-shirts in support of jailed members punk band Pussy Riot, the diverse groups of protestors marched with the chant "1-2-3, Putin go!"

Fourteen workers were killed when a building in the Moscow region caught fire Tuesday, Russia's Emergency Ministry said, adding that the victims could be Vietnamese migrant workers.

The three-story building in the town of Yegoryevsk, about 100 kilometers (60 miles) southeast of Moscow, is a former factory, a spokesman for the ministry told AFP. At lead one person was hospitalized with injuries, he added.

A passenger jet headed from New York to Moscow made an emergency landing in Iceland's capital on Thursday after receiving a bomb threat, said a spokeswoman for Russian carrier Aeroflot.

"During a flight from New York to Moscow an anonymous call was received about the existence of explosives on board the plane," the Aeroflot spokeswoman, Irina Dannenberg, told Agence France Presse.

Iran and world powers on Tuesday dug in for a second day of attritional talks in Moscow aimed at breaking an increasingly risky deadlock in the decade-long crisis over the Iranian nuclear program.

With the United States and Israel refusing to rule out military action and Tehran facing severe economic sanctions, the price of failure in the Russian capital could be high but there was no sign of progress on the first day.

Tens of thousands of protesters chanting "Russia Will be Free" rallied in Moscow on Tuesday against President Vladimir Putin's third term despite a police crackdown on their leaders a day earlier.

Flag-waving crowds made their way down leafy boulevards from Moscow's central Pushkin Square as the march began with police putting the numbers at around 18,000.

Russia on Saturday pushed its proposal for an international conference on Syria to include Iran, despite skepticism from the United States.

Russian Foreign Minister Sergei Lavrov was to give details on the plan for regional players to try to negotiate a strategy that would suit all Syrians at a briefing in Moscow at 1200 GMT.

U.S. Secretary of State Hillary Clinton called on Iran Thursday to be ready to take "concrete steps" on its disputed nuclear program ahead of a Moscow meeting between Tehran and world powers.

Iran and the so-called P5+1 group -- the United States, Britain, France, China, Russia and Germany -- meet on June 18-19 in the Russian capital to discuss Iran's nuclear program which the West fears aims to build the bomb.