Iranian President Mahmoud Ahmadinejad on Tuesday defied mass protests against his re-election by visiting Russia for a summit.

Tehran has been gripped by the biggest anti-government protests since the 1979 Islamic revolution after disputed official results showed Ahmadinejad scored a landslide in Friday's election.

Ahmadinejad arrived a day late in the Urals city of Yekaterinburg for a summit of the Shanghai Cooperation Organisation (SCO) of Central Asian powers, where Iran has observer status.

Russian President Dmitry Medvedev became the first big power leader to meet Ahmadinejad since the election and the two leaders joked and smiled for television cameras.

Ahmadinejad was shown taking part in an SCO meeting and Iranian officials said he would hold some bilateral meetings.

Demonstrators on the streets of Tehran say that the election was rigged to secure victory for the hard-line Ahmadinejad over the moderate opposition.

U.S. President Barack Obama said on Monday he was deeply troubled by the violence in Iran.

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.

Russia, which has supplied Iran with nuclear fuel for a civilian nuclear reactor, says it has been given no evidence to show that Iran is seeking to build a nuclear bomb.

Iran's president has often stolen the limelight at major conferences, including an SCO meeting in Shanghai in 2006 that was dominated by news about Tehran's nuclear programme.

Besides Russia and China, the SCO groups the ex-Soviet Central Asian republics of Kazakhstan, Uzbekistan, Tajikistan and Kyrgyzstan.

(Editing by Angus MacSwan)