Tens of thousands of people are expected to march through London calling for ambitious action to tackle climate change.
Celebrities including singer Charlotte Church and actress Emma Thompson will join the marchers, who are calling on world leaders meeting for United Nations climate talks in Paris to secure a strong and fair agreement to curb rising temperatures.
Campaigners want the Government and other countries to agree a deal that will shift the world to 100% renewable energy and protect people from the impacts of climate change.
London's march is expected to be the biggest of 2,300 events around the world this weekend, after a huge rally planned for Paris was cancelled amid security fears following the terror attacks which killed 130 people.
Ahead of the march, Church, who is performing a new song composed for the event alongside a Welsh choir, said: "Today I will march because I choose to. Tomorrow people will march across countries and continents because they have no choice but to.
"While our leaders watch on, the land, air and water that has nurtured and sustained tens of thousands of generations before us is being poisoned. Governments must be compelled to act, and it is left to us to compel them to do so."
Thompson said she is marching alongside people across the world to make "some serious noise" so leaders know people and the planet cannot wait.
"Shell has retreated from the Arctic. Coal is on the way out in many countries around the world. Now we need governments to keep up the momentum and stop propping up the dying fossil fuel industries. 100% renewables is 100% possible in the future."
The "London people's march for climate justice and jobs" will see supporters coming from as far away as the Philippines and the Arctic to join environmental groups, aid agencies, cyclists, communities affected by fracking, nurses, trade unions, climate scientists and religious leaders.
Sami reindeer herder Jenni Laiti, from Jokkmokk in the Swedish Arctic, had been planning to attend the Paris march but will now be joining other indigenous people on the streets of London instead.
She said: "We are living in the Arctic, which is one of the regions most impacted by climate change right now.
"My family are reindeer herders, our reality is that climate change isn't something coming in the future, right here and now we are totally affected by it."
She said her family need cold winters to survive but the temperatures are now fluctuating with rain falling and then turning into ice, which makes it hard for the reindeer - who dig beneath the snow to eat plants - to find food.
"We're demanding that change has to be made today, we're demanding we will have to make the change together and the world leaders will have to lead the way," she said.
"That's what we want, we want to stop climate change, we want to have a future.
"We're marching for the future of the planet, for future generations and all the people living on this planet."
Labour leader Jeremy Corbyn is due to speak at the London march, where he will say: "To tackle climate change, we must tackle global inequality, we must show that defeating the menace of global warming will not only improve the prospects of our children and grandchildren, but will improve lives here and now."
Britain should lead by example, protecting working families in high-carbon industries in the move to a low-carbon economy and investing in skills and technologies to create millions of new jobs in the green sector, he will say.
But Mr Corbyn will accuse the Tory government of refusing to lead.
"The fluffy dog has been abandoned - no more hugs for the husky instead a passionate embrace for the carbon polluters," he will say in a reference to the famous image of David Cameron with a husky on a trip to the Arctic.
"This Government is destroying the solar industry, it has removed vital safeguards to reduce the risks of fracking and cancelled support for carbon capture and storage, cut support for wind turbines."
He will tell the crowd that the Labour Party is leading on climate change.
"As we fight against global warming and the destruction of our environment, we must see it not as a burden but an opportunity - an opportunity to invest, an opportunity to create millions of new jobs, and an opportunity to create a fairer society," he is expected to say.