Green groups hoped for a big turnout at a protest in Lima Wednesday aimed at heaping pressure on ministers haggling over a post-2020 deal to curb carbon emissions.
The "People's Climate March" in the Peruvian capital follows mobilisation on the eve of the September 23 summit in New York that revived climate change as a political priority.
Hundreds of thousands of people had turned out in dozens of cities to give the summit a push.
"We're expecting between 15,000 and 20,000 people," Bert Wander of campaign group Avaaz said of Wednesday's five-kilometre (three-mile) march.
The event would draw on support from indigenous people, environmentalists and anti-poverty activists, said Wander.
Environment ministers, meanwhile, were to enter the second of a four-day meeting designed to apply political heft to the UN climate process.
The goal at the December 1-12 Lima talks is to clear the way to a pact in Paris in December 2015 that will assemble a war machine to fight climate change.
Operational from 2020, it would help ensure that global warming does not breach two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.
This is a figure that scientists say is far safer than predicted trends.
On current emission trajectories, Earth's surface will warm by some 4 C this century, a recipe for worse drought, floods, storms and rising seas.
- Window for action narrowing -
Wednesday's sessions were to focus on the thorniest issue: the format in which nations will set out promises for reducing greenhouse gas emissions.
Transparency "is the keystone" of the deal, providing the key element of trust, a European negotiator said.
Big emitters are under pressure to put their pledges on the table by the end of the first quarter of 2015, hopefully providing enough time to finalise the historic pact in Paris next December.
Other issues that must be addressed are how to ratchet up emissions curbs if -- as seems likely -- the first roster of promises fails to meet the 2 C target.
The negotiations are taking place under the banner of the UN Framework Convention on Climate Change (UNFCCC), an offspring of the 1992 Rio Earth summit.
Efforts to roll back greenhouse gases under the UNFCCC have been stymied for years by bickering and finger-pointing, driven mainly by the cost of switching out of coal, oil and gas, the backbone of the world's energy supply today.
UN Secretary General Ban Ki-moon on Tuesday urged countries not to yield to pessimism, but insisted time was running short.
"There is still a chance to stay within the internationally agreed ceiling," he said.
"But the window of opportunity is fast narrowing," he warned, adding: "We can no longer afford to burn our way to prosperity."
Ban, who has made climate change a hallmark issue of his tenure, was expected to address a meeting of Latin American countries on Wednesday.
US Secretary of State John Kerry was also expected to head for Lima.
In the runup to the conference, the United States unveiled an agreement with China on reducing the two countries' carbon emissions and pledged $3 billion (2.42 billion euros) in pre-2020 aid for climate-vulnerable poor countries.