Greta Thunberg told activists at UN climate talks in Madrid that the global school strikes have "achieved nothing".
Thunberg pointed out that despite many thousands of protests from schoolchildren across the globe, greenhouse gas emissions have continued to rise.
In the four years since the signing of the Paris accord, greenhouse gas emissions have increased by 4 per cent.
The young activist says the global strikes, sparked by her solo protest in 2017, have "not translated into action" by governments.
"I'm just an activist and we need more activists," she said. "Some people are afraid to change – they try so desperately to silence us."