Outrage: An effigy of Greta Thunberg has been found hanging from a bridge in Rome. (AP)

An effigy of Greta Thunberg has been found hanging from a bridge in Rome, prompting outrage among climate activists taking part in worldwide Extinction Rebellion protests.

The dummy, which is dressed in a yellow poncho and has the teenager's pigtail braids with a noose tied around its neck along with a sign in English saying "Greta is your God".

Italian authorities are understood to be investigating the "aggravated threat", according to reports on Monday.

Local media reports say the effigy was strung up by a group called Gli Svegli, which means “The Awake”.

View photos Local media reports say the effigy was strung up by a group called Gli Svegli, which means The Awake in English. (Twitter) More

Viginia Raggi, the mayor of Rome, condemned the “shameful” scene and tweeted that she stood in "solidarity" with the 16-year-old Swedish girl, who won a standing ovation after giving an impassioned speech at the United Nations, where she criticised world leaders for their current environmental policies.

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.

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Activists with the Extinction Rebellion movement blocked roads across major cities worldwide on Monday, kicking off a wide-ranging series of protests demanding more urgent action be taken against climate change.

Protests are planned in 60 cities worldwide, according to the group.

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.

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She said: "People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth. How dare you."

Thunberg, who is currently in the US visiting American Indian reservations in the Dakotas to talk about oil pipelines, has been tipped as a possible winner of this year's Nobel Peace Prize.

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