Package bombs have exploded at the Swiss and Chilean embassies in Rome, injuring the two people who opened them.
Police ordered checks at all embassies after a false alarm was also reported at the Ukrainian embassy.
"It's a wave of terrorism against embassies, something much more worrisome than a single attack," Rome Mayor Gianni Alemanno said.
He added that Italian authorities were following an "international path" in their investigations, distinguishing the attack from a fake bomb found earlier this week on a subway in Rome.
Italy's interior minister said investigators believe anarchists may have been behind the bombs.
Minister Roberto Maroni said the bombings were similar to the mail bombs sent to 14 embassies in Athens last month - including the Swiss - as well as to French President Nicolas Sarkozy, German Chancellor Angela Merkel and Italian PM Silvio Berlusconi.
Nobody was hurt and only two devices exploded, causing no injuries. A group called Conspiracy Nuclei of Fire claimed responsibility.
The first bomb exploded inside the Swiss embassy at around noon on Thursday. The man who opened it was taken to hospital with serious hand injuries but his life was not in danger.
About three hours later, witnesses outside the Chilean embassy said they heard a blast, sending police to the scene. One person was injured, police said.
Rome police chief Francesco Tagliente said a suspicious package at the Ukrainian embassy was a false alarm, but added that all embassies in the Italian capital had been informed of the blasts.