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, Mr 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 hit by explosions 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.
The explosion came a day after a cyberattack disrupted train services across the country, with hackers posting the phone number of the country's supreme leader as the number to call for information, state-affiliated news outlets reported.
Trains were delayed or cancelled as ticket offices, the national railway's website and cargo services were disrupted, with "unprecedented chaos at railway stations across the country", the state broadcaster IRIB reported.
A notice on electronic boards at stations asked travellers to call a number which in fact belonged to the office of Supreme Leader Ayatollah Ali Khamenei, IRIB and the semi-official news agency Fars said.
Another notice said "long delays due to cyberattacks", Fars reported.
IRIB later quoted a state railway company spokesman as saying technicians were checking the disruptions and denying that there were major delays.