Nearly 200 countries agreed at the summit to begin reducing global consumption of fossil fuel and adopt a raft of measures, including more clean energy production, to avert the worst effects of climate change.
But critics say the deal will not prevent global temperatures from rising more than 1.5C above the pre-industrial average, which scientists say will trigger catastrophic and irreversible impacts from melting ice sheets to the collapse of ocean currents.
"This text is toothless and it is nowhere even close to being sufficient to keep us within the 1.5 degree limit," Thunberg told Reuters outside Sweden's parliament where she and a handful of other protesters were calling for climate justice.
"It is a stab in the back for those most vulnerable."
The Alliance of Small Island States, which includes countries most affected by climate change like Fiji, Tuvalu and Kiribati, said the agreement was full of loopholes and was "incremental and not transformational".
Thunberg, 20, who shot to fame as the face of climate activism in 2018 after she started staging weekly protests in Sweden, said the pact was not designed to solve the climate crisis but as "an alibi" for world leaders that allowed them to ignore global warming.
"As long as we don't treat the climate crisis as a crisis and as long as we keep lobby interests influencing these texts and these processes, we are not going to get anywhere," she said.
