Climate change activist Greta Thunberg has been highly critical of global leaders over their action to limit global warming.

But speaking to the BBC's Andrew Marr ahead of the COP26 climate summit, she said she was still hopeful - but that "massive changes" were needed.

“Finally in Glasgow for the #COP26! And thank you for the very warm welcome,” tweeted the 18-year-old, who on Friday had attended a small London protest against global banks, accusing them of destabilizing the planet and putting many people’s lives at risk.

Crucial climate summit opens in Glasgow:

COP26 president Alok Sharma says the summit is the world's "last, best hope" to hit the 1.5C target.

More than 25,000 people are descending on the Scottish city of Glasgow for the two-week event, with large protests expected.

Nearly 200 countries are being asked for more ambitious targets to reduce their carbon emissions.

UK Prime Minister Boris Johnson calls it a "moment of truth" and warns that failing to act will harm future generations.

Leaders from the world's wealthiest countries, including Johnson, are currently at the G20 summit in Rome - and will travel to Glasgow later.