At the first-ever Youth Climate Action Summit at the U.N. headquarters in New York on Saturday, climate leader Greta Thunberg was joined by more than 600 young activists who were able to appeal directly to the U.N. secretary-general a day after helping to galvanize an estimated total of four million people at the Global Climate Strike—and the campaigners in attendance did not mince words.

Thunberg addressed the gathering briefly, noting that she will be speaking to world leaders at the larger U.N. Climate Action Summit on Monday.

"Yesterday millions of people across the globe marched and demanded real climate action, especially young people," Thunberg said. "We showed that we are united and that we young people are unstoppable."

"We showed that we are united." At the @UN Youth #ClimateAction Summit, @GretaThunberg highlighted the power of young people who took to the streets yesterday and who are raising their voices today for big, bold climate action.#Youth2030 #ClimateStrike pic.twitter.com/5Gig9SkO5P — UN Foundation (@unfoundation) September 21, 2019

Secretary-General Antonio Guterres thanked Thunberg and other young climate leaders from countries including Argentina, Kenya, and Fiji for helping to push for a prompt drawdown of fossil fuel emissions and investments by world governments in renewable energy.

The climate justice movement that Thunberg has helped to build in the past year has successfully convinced a number of national and local governments to declare a climate emergency and secured a commitment from the European Commission in February to dedicate a quarter of its spending to combating the climate crisis.

"You have started this movement," Guterres said. "I encourage you to keep your initiative, keep your mobilization, and more and more to hold my generation accountable."

The secretary-general added that his "generation has largely failed until now to preserve both justice in the world and to preserve the planet."

Some of the young advocates in attendance weren't convinced that the U.N. is much more committed to solving the climate crisis than it was before Thunberg captured the world's attention with the first climate strike—organized and attended solely by her in Sweden last year.

"Dear world leaders, I ask you," said one young activist. "What is the purpose of having this summit if two days from now you are letting fossil fuel corporations and CEOs of corporations take the stage along with member nations and allowing them to influence climate policy when they are the ones that created this crisis?"

"You claim you want to listen to youth solutions but this feels more like a photo op," she added.

Oil and gas executives will be holding a forum at the summit, inviting "stakeholders from across industry, academia, government, and non-profits," according to organizers, who represent British Petroleum and other corporations.

The watchdog group Corporate Accountability accused the U.N. of offering "an opportunity for some of the world's biggest polluters to greenwash" while governments remain reticent to pass laws requiring the elimination of planet-warming carbon emissions by 2050 at the latest, to avoid global warming over 1.5 degrees Celsius above pre-industrial levels.

After speaking, the young leader led other attendees in singing, "Leaders of the world, kick the oil men out, 'cause we've got no right to kill our future now."

‘You claim you want to listen to youth solutions, but this feels more like a photo op.’ — This activist led a chant calling out world leaders for continuing to work with the fossil fuel industry at the UN Youth Climate Summit Stay tuned for more of our climate week coverage pic.twitter.com/5h8aVPMBcF — NowThis (@nowthisnews) September 22, 2019

Bruno Rodriguez, an activist from Argentina, asked Guterres directly to "stop asking world leaders to just listen to science and demand they act on science."

"Enough is enough. We don't want fossil fuels anymore," Rodriguez said. "The science is clear; our world leaders have an obligation to make radical change."

Argentina Climate Activist Bruno Rodriguez speaks at UN Youth Climate Summit: "We are facing an existential emergency & our world leaders have the obligation to make radical change but change rarely takes place from the top down. It happens when millions of people demand change." pic.twitter.com/ddZdzEUfNt — The Hill (@thehill) September 21, 2019

Komal Kumar, representing Fiji, said that young people and families in frontline communities are "living in constant fear and climate anxiety."

"We demand action. Stop wasting time," Kumar said. "Engage young people in the design of adaptation plans."

"We will hold you accountable," she added. "And if you do not remember, we will mobilize to vote you out."