The UN Secretary General Ban Ki-moon has met President Ahmadinejad of Iran and supreme leader Ayatollah Ali Khameini during a visit to Tehran.
He has defied calls from the US and Israel to boycott a two-day summit of non-aligned countries.
The visit is seen as a blow to western efforts to isolate Tehran.
Opposition leaders have called on Ban Ki-moon to visit political prisoners, including Mir Hossein Mousavi and Mehdi Karroubi who have been under house arrest for 18 months.
The secretary general has indicated he intends to challenge the Iranians over human rights, terrorism and their nuclear programme.
He also wants to engage them on Syria.
“Iran has a very important role to play in helping resolve this Syrian issue, reflecting the will of the Syrian people, in a peaceful manner. The Syrian people have suffered too long, too much,” said the secretary general.
Ban Ki-moon has previously denounced Iran several times for calling for Israel’s destruction.
Tehran argues the non-aligned summit is proof that western tactics have failed.
With around 30 heads of government and delegations from a hundred countries, the gathering is described as Iran’s biggest international conference for three decades.