The puppet features long braided hair and a sign written in English reading: “Greta is your god.”
Virginia Raggi, mayor of the Italian capital, condemned the “shameful” scene. Ms Raggi also offered her solidarity “and that of the whole of Rome” to Greta and her family. “Our climate commitment does not stop,” she tweeted.
Italian authorities have opened an investigation for “aggravated threats”, the Agence France-Presse news agency reported.
Greta garnered international attention when she scolded world leaders at the United Nations Climate Action Summit last month.
The 16-year-old is currently in the United States, visiting American Indian reservations in the Dakotas to talk about oil pipelines.
The Lakota People’s Law Project said Greta is concerned about the proposed path of the Keystone XL pipeline through South Dakota, as well as plans to double oil flowing through the Dakota Access pipeline in North Dakota.
Last month, she inspired millions of young people to take to the streets of cities around the world to call for tougher and swifter steps to combat climate change.
She has also been tipped to win the Nobel Peace Prize later this week.
Additional reporting by agencies
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