The reports say a device was thrown at the convoy in the city of Hamedan, where he was to deliver a speech.
Mr Ahmadinejad was unhurt and went ahead with the speech to a rally at a football stadium, which was broadcast live on state TV.
Arab TV reports said a number of people were wounded in the attack. Dubai-based Al Arabiya television said the attacker hit a car carrying journalists. The attacker was detained, it added.
"This morning a hand grenade exploded next to a vehicle carrying reporters accompanying the president in Hamedan," Iranian website Khabaronline reported.
"Ahmadinejad's car was 100 metres (yards) away and he was not hurt."
However, Iran's state-owned Press TV has denied the attack.
An informed source in Iran's presidential office has rejected as false the reports of grenade attack on Iranian President Mahmoud Ahmadinejad.
BBC News PressTV
