Hundreds of activists have rallied in Glasgow to urge world leaders to act on climate change, in the biggest protests yet in the Scottish city ahead of the crucial COP26 summit.

Joining them was Swedish teen green queen Greta Thunberg, who arrived by train from London and was ushered from the platform and into the host city by a large police escort.

"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 destabilising the planet and putting many people's lives at risk.

The crucial UN climate conference, which runs from Sunday 31 October to Friday 12 November, is seen as crucial to addressing the threat of rising temperatures.

Activists from around the world have descended on Glasgow and organisers were expecting up to 100,000 people at a major demonstration starting on Friday.

Attendees had come from far and wide, including several other European countries, with some having walked long distances, to voice their frustrations at the UN conference starting on Sunday and running until 12 November.

Earlier, demonstrators marched through the centre of Scotland's biggest city, holding placards bearing slogans such as "actions not words" and "stop fossil fuels".

They were led by members of the Extinction Rebellion direct action group, which has brought cities around the world to a standstill with its protest tactics and has vowed to do the same in Glasgow.

"We are waiting for more ambitious measures, for our political leaders to be aware of the urgency of the situation," Dirk van Esbroeck, a Belgian pensioner, told AFP.

"Because our children, our grandchildren risk living in a much more complicated world, which is going to suffer serious climate disruptions."

He and about 20 other members of a so-called "grandparents for the climate" group had travelled by train from Belgium to Edinburgh before walking the last 75 kilometres to Glasgow.

"There is still a long way to go between declarations and action," added the 68-year-old, who has five children and 12 grandchildren.

"We just want to see concrete measures, we want to see support for the global south as well," said Becky Stockes, a 31-year-old translator who had walked from Spain.

"It's the last chance," she added of the gathering.

Student Maciej Walczuk, 19, noted that despite 25 previous COP-style summits, "The climate situation is as bad as it has ever been".

"We strive for a COP that is different... and (to) create solutions for people already affected," he said.

Representatives from almost 200 countries, including Prime Minister Scott Morrison and US President Joe Biden, are set to attend the summit, which is considered pivotal in the fight against climate change.

Ahead of the summit, Australia’s position on its future use of coal remains unclear, with Mr Morrison saying his government would not be pushed on a timeline to phase out the use of coal-fired power.

“It won't be the Australian government's policy. All countries are coming at this task from different places, their economies are different. And as a global community, we've got to understand that," Mr Morrison said after arriving in Rome on Friday evening for the Group of 20 leaders' summit ahead of COP26.

“Developing countries have different challenges to those in Europe. Indeed, Australia's economy in the shape and form of our economy is very different to many of those as well," he said.

The Morrison government this week unveiled its plan to achieve net-zero emission by 2050 but will not be lifting its 2030 target of a pollution reduction of 26 to 28 per cent on 2005 levels.

Police say about 10,000 officers from across the UK will be deployed each day during the Glasgow summit, the largest policing operation ever held in Scotland.