Investigators believe it may have been caused by two remote-controlled devices, Il Giornale reported, but the target was unclear.
The homemade devices used flammable liquid and were placed between cars parked in Via Marmorata, which links the River Tiber with the Pyramid of Caius Cestius in central Rome.
Police told reporters there was no immediate suspicion of Islamist terrorism, with officers believing the blast was an "act of protest".
Suspicion turned to anarchist groups, which have carried out similar firebombings in the past.
The perpetrators were suspected of links to an arson attack in a warehouse on the southern outskirts of Rome.
Security in the Italian capital has been heightened following a series of Isis-linked terror attacks across Europe, with the terrorist group singling out Rome as a symbolic target in its propaganda.
Letter bombs have also been found in Paris, Germany and Greece after being sent by a Greek anarchist group to EU ministers and financial institutions.
/The Independent/
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