After making it through a swarm of media cameras and microphones at the Spanish capital's northern train station, the Swedish teenager posted an ironic tweet saying that she had "successfully managed to sneak into Madrid".

"I don't think anyone saw me...," she added. "Anyway it's great to be in Spain!"

Madrid is hosting two-week, UN-sponsored talks aimed at streamlining the rules on global carbon markets and agreeing on how poor countries should be compensated for destruction largely caused by emissions from rich nations.

The talks came as scientific evidence mounts about disasters that could ensue from further global warming, including a study commissioned by 14 seafaring nations due to be published Friday predicting that unchecked climate change could devastate fishery industries and coral reef tourism.

Thunberg paid a surprise visit to the venue of the talks and joined a group of some 40 teens staging a sit-in there to demand real action against climate change.

Holding hands, the teens sang a version of John Lennon's Power To The People and displayed banners with the logo of Fridays for Future, the global climate movement inspired by Greta.

Greta did not appear unsettled by the commotion surrounding her presence.

"It's absurd. I laugh at it. I do not understand why it has become like this," the 16-year-old was quoted as saying by Swedish newspaper Aftonbladet, whose reporter rode with them in an electric car in Madrid.

"I don't like being at the centre of the focus all the time, but this is a good thing," she told Aftonbladet.

The presence in Madrid of Greta is expected to shift the attention to demands for greater action by non-governmental organisations and a whole new generation of environment-minded activists.

Past appearances have won her plaudits from some leaders - and criticism from others who've taken offence at the angry tone of her speeches.

Australian Associated Press