A day after youth-led global climate strikes, several hundred young activists including Greta Thunberg gathered for a climate summit at the United Nations on Saturday, chiding older generations for doing too little to curb carbon emissions.

The UN has invited 500 young activists and entrepreneurs to take part in the New York meeting, the first of its kind, though some were unable to attend after being denied US visas, a point raised by the organizers.

It comes days before a climate action summit which UN chief Antonio Guterres has called to seek greater commitments from world leaders on reducing their greenhouse gas emissions in line with the Paris accord to avert runaway global warming.

The tone for Saturday's event was set by an impassioned speech by Argentine activist Bruno Rodriguez, 19, who led school strikes in his native country.

"The climate and ecological crisis is the political crisis of our time, it is the economic crisis of our time, and it is the cultural crisis of our time," he said, as Guterres, who was billed as the "keynote listener," watched on.

"Many times, we hear that our generation is going to be the one in charge of dealing with the problems that current leaders have created, and we will not wait passively to become that future: The time is now for us to be leaders."

Thunberg, whose protests outside Sweden's parliament last year sparked the global youth movement, spoke first but briefly, saying she wanted to give more time to others.

"We showed we are united and young people are unstoppable," said the 16-year-old, who will also address Monday's summit.

On Friday, masses of children skipped school to join global strikes that Thunberg said were "only the beginning" of the movement.

Some four million people filled city streets around the world, organizers said, in what was billed as the biggest-ever protest against the threat posed to the planet by rising temperatures.

'Greenwashing' under fire

The corridors of the UN were filled Saturday with young people in formal suits and ties, dresses, and traditional wear from their home countries, and others wearing simple t-shirts and jeans.

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The day also saw young innovators proposing solutions, pitching their ideas to panels from leading global companies like Google.

But corporations also came under fire for their ties to the oil and gas industries.

During one testy exchange, Kathleen Ma, a 23-year-old delegate who lives in New York turned to a representative from Microsoft, which this week announced a deal with Chevron and oilfield services company Schlumberger to provide cloud computing services.

"Do you care more about getting contracts from fossil fuel companies than you care about youth? Do you care more about profits than you care about us?" she asked, to wide applause.

Lucas Joppa, Microsoft's chief environmental officer, thanked her for the question, replying: "That's one that the entire tech sector, and everybody in the world we live in today which is predicated upon an oil and gas economy has to answer.

"It's one that you'll be hearing more about both from Microsoft and our peers in the broader tech sector as this moves forward."

Rodriguez, the activist from Argentina, later told AFP that young activists were strongly in favor of efforts to divest from the fossil fuel industry, which was responsible for "pillaging" across Latin America.

He added that he welcomed the fact that the corporate sector had a heavy presence on the sidelines of the main UN climate summit this week, but that their efforts need to go beyond rhetoric and "they also need to be conscious that the greenwashing speech has no place anymore."

A landmark UN report to be unveiled next week will warn global warming and pollution are ravaging Earth's oceans and icy regions in ways that could unleash misery on a global scale.

But Guterres struck a more optimistic note Saturday, crediting young activists with spurring action.

"This changing momentum was due to your initiative and to the courage, with which you have started these movements."