Swedish climate activist Greta Thunberg joined other young activists outside the United Nations headquarters Friday to call for more action to combat climate change.

The 16-year-old joined several dozen other activists who waved placards and gave speeches.

One speaker criticised both US political parties - the Democrats and the Republicans - for taking political donations from fossil fuel companies.

Thunberg arrived Wednesday after crossing the Atlantic on a carbon-free sailboat and is due to attend the UN Climate Change Summit in September.

She arrived in New York Harbour to cheers as she stood on the bow of the boat and waved a red flare.

"All of this is very overwhelming and the ground is still shaking for me," Thunberg said during a press conference later.

"It is insane that a 16-year-old should have to cross the Atlantic ocean to make a stand," she added.

Thunberg attracted international attention last year when she started missing school on Fridays to protest against political inaction on climate change, inspiring youth around the world to do the same.