Rome was on high alert on Christmas Eve after parcel bomb blasts left two staffers at the Chilean and Swiss embassies in the Italian capital badly injured in an attack claimed by "anarchists".

Embassies, ministry buildings, post offices and the parliament were under tighter security after the two explosions on Thursday and false alarms in three other foreign embassies and two local government buildings in Rome.

The Chilean injured as he opened a package delivered to the embassy lost two fingers and was wounded in the chest and one eye, hospital sources said. It is feared the Swiss embassy worker wounded in the same way could lose his hand.

Chilean Foreign Minister Alfredo Moreno said the attack was "cowardly" and "an act of terrorism," while Swiss Foreign Minister Micheline Calmy Rey condemned "such malicious and unforgivable acts in the strongest terms."

Both bombs were contained inside plastic VHS video boxes packed with gunpowder and metal fragments connected to a nine-volt battery and a light bulb filament, La Repubblica newspaper reported, citing investigators.

An insulating tape kept the bombs from going off until they were opened.

A group calling itself the "Informal Federation of Anarchy, or FAI under its Italian acronym, claimed responsibility in a blackened note found at the scene of the Chilean embassy blast amid fragments of the package that exploded.

The FAI has carried out a number of small attacks in Italy in recent years.

"We have decided to make our voice heard with words and deeds. Let us destroy the system of domination... Long live anarchy," the statement said.

The note also expressed solidarity with "comrades in prison" and fellow anarchist groups in Argentina, Chile, Greece, Mexico and Spain.

The statement was signed off by the FAI's "Lambros Fountas Cell" -- a reference to a Greek anarchist killed in a firefight with police in March 2010.

"The targets were not chosen at random," Alfredo Mantovano, a junior interior minister, was quoted as saying by Il Giornale daily.

Mantovano said Switzerland was chosen because of the arrest of some Italian anarchists there and Chile because an anarchist died there in a bomb blast last year -- a death blamed by anarchist groups on Chilean authorities.

Violent anarchist groups in Italy have "a few hundred members" under a variety of banners, Mantovano said, adding that "their activities are kept under careful scrutiny by secret services and Digos" special forces.

Italian investigators drew parallels between Thursday's bombs and a suspected anarchist far-left plot in Greece last month in which bombs were sent to foreign embassies in Athens and European government leaders.

Over a dozen packages containing explosives were sent in that plot, prompting Greece to suspend international mail for two days. At least four of the packages ignited or exploded, slightly injuring one person.