Italian police have been searching embassies and consular offices in Rome after parcel bombs exploded at the Swiss and Chilean missions.
A Swiss man suffered serious hand injuries while a Chilean man was slightly hurt.
Both were members of staff and had been opening packages at their respective embassies when the devices exploded.
An Italian anarchist group said it was behind the attacks in a note found on the clothing of the injured Chilean.
The message, written on behalf of the Informal Anarchist Federation (FAI), said: "We have decided to make our voice heard with words and with facts, we will destroy the system of dominance, long live the FAI, long-live Anarchy."
The Italian intelligence services said in a report to parliament last year that the FAI was "the main national terrorist threat of an anarchist-insurrectionalist type".
Prosecutors have opened an inquiry for an attack with "terrorist" intent.
A suspect package was also reported at the Ukrainian embassy but no dangerous items were found, the embassy told the Reuters news agency.
A spokeswoman for the Swiss embassy said the explosion there had occurred around midday (1100 GMT).
The embassy is located on via Barnaba Oriani near Villa Glori Park, to the north of the city centre.
Some time between 1400 and 1500, a package exploded at the Chilean embassy on the via Po near the Villa Borghese in central Rome. A Chilean member of staff sustained minor injuries.
The package had been addressed to a cultural association, Chilean Foreign Minister Alfredo Moreno said.
Before the FAI note was published, Interior Minister Roberto Maroni said Italian investigators were pursuing the "trail of anarchists-insurrectionists".
"Various elements lead us to think that this is the correct path," he added without giving details.
"These are very violent groups that are also present in Spain and Greece and are very well connected."
Parcel bombs were sent to embassies in the Greek capital Athens last month.
More than a dozen packages were sent, prompting Greece to suspend international mail for two days.
At least four of the packages ignited or exploded, slightly injuring one person.
'Wave of terrorism'
Responding to Thursday's blasts, the Mayor of Rome, Gianni Alemanno, said: "It's a wave of terrorism against embassies, something much more worrisome than a single attack."
It is unclear if the parcel bombs are connected to a suspected bomb found on an empty underground train in Rome on Tuesday.
The device lacked a detonator and tests showed it contained no explosive.
Anti-government protests have gripped Italy over the past week, with Rome the scene of some of the most serious rioting.