An AFP journalist saw the iconic Swede -- who has previously been detained for civil disobedience at protests in various countries -- carried away by police after refusing to leave a sit-in.

Thunberg, 21, was part of a smaller group of demonstrators who broke away from a march organised by the United for Climate Justice movement that began outside the European Parliament.

The rally was calling for the European Union -- headquartered in Brussels -- to end subsidies for fossil fuels in order to achieve its ambitious goal of making the continent carbon neutral by 2050.

"This will not happen without an immediate phase out of fossil fuel subsidies," climate activists, scientists and economists wrote in an open letter to EU leaders.

"Until these necessary changes occur, people will continue to take to the streets to make our voices heard and hold you accountable."