'Build back better. Blah, blah, blah. Green economy. Blah blah blah. Net zero by 2050. Blah, blah, blah,' the 18-year-old activist said during a keynote address to the Youth4Climate meeting in Milan.
Thunberg, who rose to fame thanks to her 'school strike for climate' protests in her native Sweden, added: 'This is all we hear from our so-called leaders.
'Words that sound great but so far have not led to action. Our hopes and ambitions drown in their empty promises.'
Greta Thunberg mocked world leaders and their 'empty promises' on climate change when she addressed a youth summit in Milan today
The 18-year-old activist quoted from Boris Johnson and Indian PM Narendra Modi before mocking their pledged by saying: 'Blah, blah, blah'
She singled out Mr Johnson for mockery in her speech, quoting his words that climate action is 'not some expensive, politically correct... act of bunny hugging.'
Indian Prime Minister Narendra Modi also found his words thrown back at him, as Greta quoted him saying: 'Fighting climate change calls for innovation, cooperation and willpower.'
The Youth4Climate event is a three-day summit attended by 400 young representatives from 190 nations which will be used to develop climate policies.
It runs until September 30, which is the first days of the Pre-COP conference - a summit of energy and climate ministers from around the world in preparation for the full COP26 meeting in Glasgow later this year.
On the final day of the youth summit, the young representatives will present their policies to ministers - with the best taken to Glasgow for discussion by world leaders.
Greta arrived at the summit after a seven-hour train journey from Frankfurt - where she has been pressuring Germany's election candidates over climate change.
She was swarmed by reporters at the station, while she waited to take a mandatory Covid test.
When asked what she was expecting from the talks, Greta gave a typically-downbeat assessment - replying 'not a lot' before adding that it will be 'just like any other meeting, with lots of talking.'
Thunberg was addressing the Youth4Climate portion of the Pre-COP conference, which is the last formal meeting between climate energy ministers from around 50 nations ahead of the main COP26 summit in November.
Thunberg was opening the Youth4Climate summit in Milan today which encourages youth