“We children are not sacrificing our education and our childhood for the adults and politicians to tell us what they consider is politically possible in the society they have created,” the 16-year-old told a crowd of around 25,000 demonstrators in the Italian capital.
“We have not taken to the streets for them to take selfies with us, and tell us that they really, really admire what we do.
“We children are doing this to wake the adults up.
“In the last six months, millions of school children, not least in Italy, have been school striking for the climate. But nothing has changed. There is no political change whatsoever in sight.
“That is why we must prepare ourselves. This will not take weeks, this will not take months, this will take years.”
Teenage pedal power was used to charge the Rome stage where Thunberg gave her speech.
Around 128 bicycles were rigged up to a dynamo and generator in Piazza del Popolo.
Thunberg arrived in Italy on Wednesday, meeting Pope Francis at the Vatican shortly after touching down in Rome.
The pontiff encouraged Thunberg to carry on with her mission.
“Continue, continue,” Francis told the teen.
“Go on, go ahead.”
Thunberg responded: “Thank you for standing up for the climate, for speaking the truth. It means a lot.”
Thunberg also addressed the Italian Senate on Thursday, appealing to lawmakers for more awareness and immediate answers to the pressing issue of climate change.
“This is the most difficult crisis we have ever had to face, and also the simplest one,” she said.
“It is the simplest, because we do know what we have to do; and it is the most difficult, due to the economic aspects of the use of fossil fuels.”
Following her trip to Italy, Thunberg travelled to the UK to continue spreading her message.