Their proposals will be vetted by climate and energy ministers who are gathering at the same venue in Italy's financial centre for their pre-COP26 meeting, and the best will be sent on for consideration at the summit in Glasgow, Scotland.

"This young generation of activists have changed the politics," Nigel Topping, Britain's high level climate action champion, told Reuters in reference to the commitments some countries have made to reducing carbon emissions ahead of the COP26 meeting, which is being co-hosted by Britain and Italy.

The meetings come as soaring energy prices on world markets stoke fears of a popular backlash against climate reform.

"They are a really important voice and it's important that voice gets focused, because just saying 'you have to do more' only goes so far," Topping added in an interview.

Money speaks

At their pre-COP26 summit, which starts on Thursday after the Youth4Climate meeting, some 50 climate ministers will tackle political hurdles ahead of Glasgow, including different views on the necessary pace of transition and who pays for it.

While new energy and funding pledges from the US and China have left negotiators more upbeat, many G20 countries, including major polluters like China and India, are yet to deliver updates of their short-term climate action plans.

"The issue is on the agenda and could cast a shadow," one source involved in the Milan talks told Reuters.

Climate activists are demanding policymakers match rhetoric with action and stump up the billions of dollars needed to wean the world off fossil fuels to cleaner energy during a year that has seen record-breaking heatwaves, floods and fires.

"Money speaks, and if rich nations don't restructure debt for poor nations and commit $500 billion for climate action from 2020-2024, there's no point in wasting time at these meetings," Oscar Soria of the US-based activist network Avaaz said.

While nothing concrete in terms of numbers was expected from Milan, the political signals and the manifesto of the activists would set the tone for Glasgow, the source involved in the talks said.

Wealthy nations who pledged a decade ago to mobilise $100 billion a year to help vulnerable countries adapt and transition to cleaner energy are still short of their 2020 goal.