A loud blast heard in north Tehran early on Saturday was caused by an "unknown object" exploding in a park but no-one was hurt, Iranian state television reported.
"An unknown object exploded ...There was no damage and no one was hurt," a reporter said.
Tehran Deputy Governor Hamid Reza Goudarzi, who is in charge of security issues, visited the site.
"Just one explosion took place inside Mellat (People) Park," he told the semi-official Tasnim news agency.
Asked whether the incident was an attack, Goudarzi said: "We are investigating the dimensions and causes of the incident and we will provide information after we are sure."
Attacks are rare in Iran, but a number of sensitive military and nuclear sites have been targeted in recent years.
Iran has accused Israel of several attacks on facilities and scientists linked to its nuclear programme. Israel has neither denied nor confirmed the allegations.