Iranian officials have denied that President Mahmoud Ahmadinejad was targeted in an assassination attempt.
A conservative Iranian website had said an explosive device was thrown at his convoy in the western city of Hamedan, where he was to deliver a speech, but later removed the report from its site.
State-run Press TV said "no such attack had happened", and officials said the blast was caused by a firework.
Mr Ahmadinejad went ahead with his speech at a football stadium.
"It was a firecracker, and a statement will be released soon," an official in the president's media office told the Agence France-Presse news agency.
Al Alam, a state-run Arabic-language TV channel, reported that the firecracker was set off to cheer the president.
The Khabaronline website said the president's car was about 100 metres (330ft) away from the explosion, which "caused a lot of smoke", but later removed the article from its website.
The semi-official Fars news agency first said the device was a homemade grenade, but later said the explosion was caused by a firecracker.
Earlier, a source in his office told the Reuters news agency that Mr Ahmadinejad's convoy was targeted as he was travelling from the airport.
Dubai-based Al Arabiya television said the attacker threw an explosive device at a car carrying journalists.
The attacker was detained, it said.
Other Arab TV reports had said a number of people were wounded in the attack.
Earlier this week, Mr Ahmadinejad said he believed he was the target of an assassination plot by Israel.
During a speech to a conference of expatriate Iranians in Tehran, he said Zionists had "hired mercenaries to assassinate me".