Teenage climate activist Greta Thunberg has captured the world's attention in a fierce and passionate speech at the United Nations headquarters, accusing world leaders of failing to act on climate change.

Key points: Greta Thunberg told the UN summit that even the strictest emission cuts being talked about were "not good enough"

Greta Thunberg told the UN summit that even the strictest emission cuts being talked about were "not good enough" The teenage activist told world leaders that her generation would "never forgive" them for inaction on climate change

The teenage activist told world leaders that her generation would "never forgive" them for inaction on climate change She stared down US President Donald Trump when they crossed paths at the UN headquarters

The Swedish campaigner's brief address electrified the start of the Climate Action Summit in New York.

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The UN event was aimed at mobilising government and business to break international paralysis over carbon emissions, which hit record highs last year despite decades of warnings from scientists.

"This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean, yet you all come to us young people for hope. How dare you," said the 16-year-old, her voice trembling with anger at times.

"You have stolen my dreams and my childhood with your empty words.

"We are in the beginning of a mass extinction and all you can talk about is money and fairy tales of eternal economic growth."

Ms Thunberg told the UN that even the strictest emission cuts being talked about only gave the world a 50 per cent chance of limiting future warming to another 0.4 degrees Celsius from the current level, which would meet the global goal of 1.5C — the benchmark temperature rise laid out by the 2015 Paris Agreement that will limit the impact of warming on world weather systems.

Those odds were not good enough, she said.

"We will not let you get away with this," Ms Thunberg said.

More than 50 global leaders — with the exception of Australian Prime Minister Scott Morrison and US President Donald Trump — were set to take part in the summit by announcing climate finance measures and transitioning from coal power.

Mr Trump dropped in briefly and listened to one of the speakers. Ms Thunberg stared down the US President when the two crossed paths.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 27 seconds 27 s Greta Thunberg eyes Donald Trump as he enters UN

Leaders were only permitted to speak at the event if they could offer up new climate action plans.

Inspired by Ms Thunberg's solitary weekly protest outside the Swedish Parliament a year ago, millions of young people poured onto the streets around the globe last Friday to demand governments attending the summit take emergency action.

After her speech, Ms Thunberg and 15 other young climate activists filed a complaint with the United Nations Committee on the Rights of the Child alleging that world leaders' inaction on the climate crisis had violated children's rights.

The petitioners, who range in age from eight to 17 and hail from 12 different countries, teared up as they presented their complaint at the United Nations Children's Fund (UNICEF) headquarters and gave personal accounts of how their lives and homes have been upended by climate change because of politicians' inaction.

The complaint accuses the respondent countries — Argentina, Brazil, France, Germany and Turkey — of knowing about the impact of their carbon emissions on the climate and doing nothing to mitigate it.

Donald Trump did not speak at the summit, but briefly listened in to one of the speeches. ( AP Photo: Evan Vucci )

The respondents are a few of the biggest carbon emitters out of the 45 countries that have signed a protocol allowing children to seek redress under the 1989 Convention of the Rights of the Child, a treaty that declared the unassailable civil, economic, social, political and cultural rights of children.

Other major carbon emitters such as the US and China have not signed the protocol.

'The biggest cost is doing nothing'

Global warming caused by heat-trapping greenhouse gases from burning fossil fuels has already led to droughts and heatwaves, melting glaciers, and rising sea levels and floods, and scientists say the crisis has intensified since world leaders signed the 2015 Paris Agreement to combat climate change.

A new report from the World Meteorological Organisation released ahead of the conference warned the world was falling far behind in the race to avert a climate disaster.

The last five years have been the hottest on record, the report said, with ice sheets melting and sea levels rising at an unprecedented rate.

"I was very struck by the emotion in the room when some of the young people spoke earlier," French President Emmanuel Macron told the summit.

A cohort of leaders from the Pacific, which has borne the brunt of the climate impacts such as extreme weather in recent years, also warned the summit that enough was not being done to stave off climate change, calling it a "living nightmare".

"I think that no political decisionmaker can remain deaf to this call for justice between generations."

UN Secretary-General Antonio Guterres, who organised the one-day event, had warned leaders only to turn up if they came armed with concrete action plans, not empty speeches.

"There is a cost to everything. But the biggest cost is doing nothing. The biggest cost is subsidising a dying fossil fuel industry, building more and more coal plants, and denying what is plain as day: that we are in a deep climate hole, and to get out we must first stop digging," he said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 36 seconds 36 s UN chief says leaders have an obligation to stop the climate crisis.

Nevertheless, there were few new proposals from governments for the kind of rapid change climate scientists say is now needed to avert devastating impacts from warming.

The summit has, by contrast, been marked by a flurry of pledges from business, pension funds, insurers and banks to do more.

"There will not be any solutions or plans presented in line with these figures here today because these numbers are too uncomfortable and you are still not mature enough to tell it like it is," Ms Thunberg said.

"The eyes of all future generations are upon you, and if you choose to fail us, I say we will never forgive you."

"We will not let you get away with this," teenage activist Greta Thunberg told world leaders in New York. ( Reuters: Lucas Jackson )

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