Iran's president has told Central Asian, Russian and Chinese leaders they should co-operate to prevent what he called threats from domineering powers.
Mr Ahmadinejad is probably the most significant presence
Addressing the SCO group of countries, Mahmoud Ahmadinejad said it could become a strong institution if it extended ties to the political sphere.
Correspondents say the speech is likely to heighten concerns that the group is emerging as an anti-US bloc.
Mr Ahmadinejad is attending the week-long meeting as an observer.
The issue of Iran's nuclear programme is likely to overshadow the meeting of the group, known as the Shanghai Co-operation Organisation (SCO).
The United States has criticised the SCO for inviting the Iranian leader to the gathering.
Members China and Russia have resisted pressure for UN sanctions against Iran over the programme.
Leaders are also expected to discuss terrorism and the drugs trade.
The SCO was originally formed to resolve frontier problems between China and its post-Soviet neighbours.
But the BBC regional analyst Michael Haslett says the group now has ambitions to become a major force in Asian politics.
A number of non-member states have been invited to this week's meeting - including the West's ally President Hamid Karzai of Afghanistan.
But our correspondent adds that the most significant presence is probably that of Mr Ahmadinejad.
Major powers have offered incentives to Iran if it stops uranium enrichment.
Western diplomats have signalled satisfaction with Iran's initial response to the plans, which may allow Tehran to enrich uranium in the future.
The SCO takes in China, Russia, Kazakhstan, Kyrgyzstan, Tajikistan and Uzbekistan.
Four nations - Iran, Mongolia, India and Pakistan - have observer status.
Analysts say both Russia and China have become increasingly concerned about the presence of the US in Central Asia.
Last year the SCO raised the issue publicly, calling for the withdrawal of US military bases from Uzbekistan and Kyrgyzstan.