It's "now or never" when it comes to climate change, according to Pope Francis, likening inaction to suicide.

"Every year the problems are getting worse. We are at the limits. If I may use a strong word I would say that we are at the limits of suicide," he said.

The pope, who wrote a major document on the environment last June, made the comment aboard the plane returning him to Rome at the end of his six-day trip to Africa.

He spoke of retreating glaciers in Greenland and low-lying countries at risk from rising sea levels.

"I am sure that the [Paris delegates] have goodwill to do something. I hope it turns out this way and I am praying that it will," he said.

World leaders launched an ambitious attempt on Monday to hold back the earth's rising temperatures, with the United States and China - the world's biggest carbon emitters - urging the UN climate summit in Paris to mark a decisive turn in the fight against global warming.

The pope's last stop in Africa was the Central African Republic, one of the continent's poorest nations.

"Africa is a victim," he said. "Africa has always been exploited by other powers...there are some countries that want only the great resources of Africa.

"But they don't think about developing the countries, about creating jobs. Africa is martyr, a martyr of the exploitation of history," he said.

Religious leaders in London united to address the UK's largest ever climate change march on Sunday, calling for the British government to step up its response to the impending climate crisis.

Speaking together at the rally, Rabbi Laura Janner-Klausner (senior rabbi to Reform Judaism), Dr Ruth Valerio (churches and theology director, A Rocha UK) and Shanza Ali (director of Muslim Climate Action) told marchers:

"Our relationship with the earth is like our relationships with each other. Every relationship, every person, every life, is precious, and a gift from God that we must treasure. So too we must treasure our relationship with the world. We shoulder the responsibility that comes with stewardship over this planet.

"Right now, something irreversible is happening, something that will destroy the gift we've all been given, Christian, Muslim, Jew, people of any faith and none. Our earth is gathering scars and scratches from overuse and abuse. There's no insurance policy. We can't replenish lakes and trees, oil and minerals, melting ice caps. But together, we can halt this damaging process."

The march, which was 50,000 people strong was preceeded by an interfaith prayer event at Westminster Synagogue led by diaspora communities from countries adversely affected by climate change.

Additional reporting by Reuters.