An effigy of teenage climate activist Greta Thunberg strung up under a bridge in Rome has prompted Italian authorities to open an investigation for "aggravated threats", media reported Monday.

Images posted on social media showed an effigy with long braids with a sign written in English reading "Greta is your God".

Politicians slammed the stunt with Rome's mayor Virginia Raggi expressing the city's "solidarity" with the Thunberg family in a message on Twitter.

Leftist PD party leader Nicola Zingaretti condemned the "grisly violence" and said the party does not respect the values shared by those responsible for the effigy.

The 16-year-old Swedish activist hit international headlines after she launched a "Fridays For Future" campaign last year, calling for students across the globe to protest outside their parliaments to demand climate action.

Hundreds of thousands of school children and adults heeded Thunberg's call last month in a global strike ahead of a UN climate summit.

She has been tipped as a possible winner of this year's Nobel Peace Prize.