ROME (REUTERS) — A small explosion rocked central Rome, alongside the Interior Ministry, in the early hours of Tuesday, destroying several small vehicles and blowing out windows but causing no injuries, firemen said.

Small explosion hits Rome, even under heightened securityThe blast occurred at about 4 a.m. (10 p.m. EST Monday). The damage appeared to be localized, according to a Reuters witness who was at the scene shortly afterwards.

“This was an explosion, not a fire,” a member of the fire brigade told Reuters, but said it was too early to give any other indications as to what caused it.

The explosion was heard across the city center, sending residents pouring into the streets before police arrived and urged onlookers back into their homes.

Police said they were removing glass from apartment buildings next to the Interior Ministry, which sits on a low rise in the center of the city.

A building housing offices of the Democrats of the Left — Italy’s largest opposition party — stands in the street where the blast took place, but there was no evidence it had been targeted.

Rome has been under heightened security in the past week following the detention of a group of Moroccans found with a cyanide compound and explosive material. The men are suspected of having plotted a possible terror attack against the U.S. embassy in the city.

In recent months the country has also been hit by several small explosions, most prominently in Venice. These appeared to be motivated by domestic politics rather than international affairs.