Thousands of demonstrators marched in nations around the world on Sunday ahead of this week's UN Climate Summit, a meeting environmental groups think could help move the world towards a new global agreement to reduce greenhouse gas pollution.

More than 125 world leaders at the summit on Tuesday, the largest number of heads of government to ever attend a climate summit. Canadian Prime Minister Stephen Harper, who will be in New York, will not attend. Canada will be represented by Environment Minister Leona Aglukkaq.

The summit has been convened by UN Secretary General Ban Ki-Moon as way to generate momentum for next year's climate conference in Paris. The hope is that Tuesday's summit will see leaders outline their plan of action for next year that could result in a new climate deal.

"That meeting will play a critical role in the Paris agreement," said Tim Gray, executive director of the group Environmental Defence.

There's been a lot of global foot-dragging over crafting a new climate agreement in the last few years, but environmental groups say next week could change that.

Canadians join New York march

A key reason for their optimism is the response to the People's Climate March in New York on Sunday leading up to the summit.

About 100,000 people marched through the streets to press for action on climate change. Luminaries included Ban, actors Leonardo DiCaprio and Mark Ruffalo, former U.S. vice president Al Gore, New York Mayor Bill de Blasio and primatologist Jane Goodall.

Among them were throngs of Canadians, with scores from one Toronto environmental group alone hopping on busses for a 12-hour trek.

Also on hand was Sierra Club Canada program director John Bennett, who said it was long past time for Harper to take concrete steps to reduce the country's greenhouse gas emissions.

"Even though he's in control in Canada he's never respected the demand for action on climate change that the population has made. Canadians overwhelmingly want action on climate change and he has not delivered on that.

"We want to bring a message to the United Nations and to Mr. Harper that we want to see some real action to reduce our emissions and that means stop betting on fossil fuels and doing things to reduce our emissions, and to help the rest of the world do the same," Bennett said.

Seventy communities across Canada planned similar events.

In London, organizers said 40,000 marchers took part in an event, while a small gathering in Cairo featured 15-metre art work representing wind and solar energy.

World leaders will convene in New York this week to lay out plans for a new agreement to curb greenhouse gas emissions. (Peter Andrews/Reuters)

The sheer size of the New York event was unprecedented, according to environmental activist Tzeporah Berman.

"We've seen a dramatic rise of extreme weather," Berman said in an interview with CBC News.

"So now people are starting to see and feel the impacts of climate change and that's why you see so many buses of Hurricane Katrina survivors coming to this march and, in fact, impacted communities and indigenous communities are leading this march."

And it's not just environmental activists. U.S. senators and Canadian politicians including Green Party Leader Elizabeth May and Ontario Minister of the Environment and Climate Change Glen Murray were there.

Murray marched "to emphasize the importance of co-operation and collaboration with our partners if we are to overcome the challenge before us," according to a statement from his office.

Canada's controversial climate legacy

But the momentum will need to continue as political leaders get down to business on Tuesday.

U.S. President Barack Obama will address the summit during the day-long event.

And while his speech will draw major attention, Canada could come in for some serious criticism over the continued lack of national regulations to control greenhouse gas emissions from the country's largest source of carbon pollution — the energy industry.

"Despite repeated promises since 2008, Canada has given the oil industry a free pass," Tim Gray said in a briefing with reporters. "There is not a single federal law or regulation addressing carbon pollution from the tar sands or any other oil and gas facility in the country."

Obama looks to craft global deal

In contrast, the U.S. has introduced proposed measures to curb its largest source of carbon pollution — coal-fired power plants.

Obama sees action on climate as his legacy and the summit this week could outline his approach to crafting a new global deal, according to Jake Schmidt, international climate policy director at the Natural Resources Defense Council in Washington.

"He will be speaking to the world to say 'Yes I'm serious about dealing with this challenge and here are the set of measures I'm doing to deliver upon our commitments,'" Schmidt said.

But the bottom line is the summit will be a partial success if leaders can simply avoid the sniping and finger-pointing that has marred previous climate gatherings.

Berman predicts a shift in attitude toward climate change and its effects may play a role this time.

"It's not just scientists and environmental groups calling for these changes now," said Berman. "It's all of these conservative bodies. It's banks, it's world governments, it's the International Energy Agency."