MrMohammadi, a lecturer at Tehran university, died when a bomb strapped to a motorcycle was triggered by remote control outside his home in the northern Tehran neighbourhood of Qeytariyeh, state media said.
Iran's state broadcaster said Mr Mohammadi was "assassinated in a terrorist act by counter-revolutionaries and agents of the arrogance," without naming any sources.
Iranian officials usually refer to the United States and some other Western powers as the "global arrogance".
Tehran prosecutor Abbas Jafari Dolatabadi told the ISNA news agency that Mr Mohammadi was a lecturer in nuclear energy and said the booby-trapped bike was parked outside his house and exploded as he was getting into his car.
Bomb attacks are rare in Iran although several security officials and members of the elite Revolutionary Guards have been killed in bombings by rebels in restive Sistan-Baluchestan province in eastern Iran.
A witness said today's explosion was a "strong blast breaking window glasses in neighbouring houses and cars."
Iran's state-run Arabic-language TV Al-Alam identified Mohammadi as a "hezbollahi" teacher -- a term used for staunch supporters of the Iranian regime.
"This assassination may have been carried out by the Hypocrites (Iran's exiled People's Mujahedeen opposition) or planned by the Zionist regime," Al-Alam said.
Iranian authorities have consistently accused arch foes the United States and Israel of seeking to ferment unrest in Iran.
Hardliners have also accused the People's Mujahedeen of infiltrating anti-government protests and carrying out attacks.