Barack Obama meets Greta Thunberg at the Obama Foundation headquarter in Washington DC on Tuesday. The Obama Foundation / AFP

Activism works.  ‪So act. ‪See you in the streets! These were the words of  Swedish teenage climate activist Greta Thunberg.

She urged everyone to join her ‪Sept.20-27 in the streets to make a difference.

Thunberg offered a blunt message to Congress on Tuesday as she brought her campaign for urgent action on climate change to the US Capitol.

"I know you're trying," she told Democratic senators at an invitation-only forum, "but just not hard enough. Sorry."

Massachusetts Senator Ed Markey thanked the 16-year-old activist for her advice and her activism, which has gained worldwide attention by inspiring a series of protests and school strikes, including one set for Friday.

Thunberg and other young activists bring "moral clarity" to the fight against global warming, Markey said.

"We hear you," he told her, vowing that lawmakers "will redouble our efforts to make sure that we inject this issue into the politics of this building and this country because time is running out."

Markey and other lawmakers hailed Thunberg as a "superpower," noting that her activism has drawn a passionate following of children essentially challenging their elders to take action.

"Save your praise," Thunberg replied.

"We don't want it," she added, especially if officials intend to talk about climate change "without doing anything about it."

Thunberg was in Washington ahead of a global strike planned for Friday.

Activists are calling for immediate action from the world's governments to halt global warming, reduce fossil fuel consumption and avert environmental catastrophe.

Instead of listening to her and other teenagers, lawmakers should invite scientists to the Capitol to listen to their expertise on ways to slow a rise in global temperatures, Thunberg said.

"This is not about us. This is not about youth activism," she said.

"We don't want to be heard. We want the science to be heard."

Despite Thunberg's request, lawmakers bombarded her and other youth activists with praise, saying they had sparked a global movement that is already being felt in the 2020 presidential campaign and in the halls of Congress, where lawmakers are debating proposals such as the Green New Deal.

Markey is a co-sponsor of the Green New Deal, which would shift the US economy away from fossil fuels such as oil and coal and replace them with renewable sources such as wind and solar power.

"We need your leadership," he told Thunberg and other activists.

"It's creating a new X-factor" to boost efforts to fight climate change.

Barack Obama met Greta Thunberg in Washington, the former US president said on Tuesday.

Obama announced the Monday meeting via Twitter, hailing the 16-year-old as "one of our planet's greatest advocates."

The Obama Foundation released a video of the former president's meeting with Thunberg -- who is visiting the United States after crossing the Atlantic in an emissions-free boat.

"No one is too small to have an impact and change the world, so just do everything you can. Be creative," Thunberg said during the meeting.

"You and me, we're a team," said Obama, before asking the young activist if she believed in fist bumping, to which she responded in the affirmative.

Agencies