Between 50,000 and 100,000 people are expected to mobilise in Glasgow as part of a global day of action for climate justice.
First Minister Nicola Sturgeon urged people to attend and exercise their right to peaceful protest.
She said: “Come out, make your voice heard, do so peacefully. It is a powerful agent for change, and in the climate context, the United Nations actively encourages peaceful protest as part of the COP process, so [I’m] a big supporter of that.”
Dozens of roads across the city centre and west end have been closed from 5am, with possible disruption spreading to air passengers after activists threatened unspecified action at or near Glasgow Airport.
Crowds will assemble at midday at Kelvingrove Park before marching to Glasgow Green via the city centre for a rally at 3pm.
A feeder march will set off from Queens Park in the south of the city at noon and link up with the main crowd in George Square, before heading on to Glasgow Green.
Thirteen coachloads of protesters are due from the capital, with the transport organised by the Edinburgh COP Coalition.
Mikaela Loach, a young Scottish climate activist who is challenging the UK's North Sea oil and gas expansion in court, is among those taking part today.
She said: "Many thousands of us are marching right across the world today to demand immediate and serious action.
"We're clear that warm words are not good enough - and that the next week of talks must see a serious ramping up of concrete plans."
The marches come after thousands of youth activists, including Greta Thunberg and Vanessa Nakate, marched through Glasgow on Friday protesting against investment in fossil fuels and failure to tackle the climate crisis.
Ms Thunberg called the COP26 conference, where countries are meeting in a bid to increase ambition on cutting greenhouse gas emissions, " a global north greenwash festival, a two-week long celebration of business as usual and blah blah blah".
But US special presidential envoy for climate John Kerry has said there is a greater sense of urgency and focus at the COP26 talks than ever - though he said he was "one of the frustrated" over the pace of climate action and warned that it was "not job done" at the conference.
Today, demonstrators will also be on the streets of central London, as well as for 200 events across the UK and around the world, organisers said.
The latest demonstrations come midway through the COP26 summit, which has seen about 120 leaders gather in Glasgow to set out the action they are taking to cut deforestation, coal, funding for fossil fuels abroad and methane emissions.
But there is still a significant gap between the measures countries have committed to and what is needed to avoid more than 1.5C of warming, beyond which the worst floods, droughts, storms and rising seas of climate change will be felt.
As the protests take place, negotiations continue at COP26, while the conference is also focusing on the role of nature, land use and agriculture in tackling climate change on Saturday.