Environmental group Extinction Rebellion or XR kicked off the event on Friday, promising less disruption and more inclusion than the blockades that became its trademark.

The group says thousands of people protested outside government departments in London on Friday “to highlight the environmental and social failures across them all,” according to XR.

Saturday’s protest focused on nature and biodiversity, and started from Westminster Abbey with attendees, many of them children, wearing animal costumes and masks.

“It’s an emergency. Everybody needs to pull together so the future generations can enjoy our beautiful planet,” said 47-year-old Jenny O’Hara Jakeway, who made the six-hour journey from Wales with her two children.

“I should protest more but my life is work and family. Being passive is not an option anymore because of the urgency of the situation,” she said.

Many had made banners for the occasion, with one reading: “We defend the climate but police arrest us” and another “Extinction is forever.” Others warned that a third of UK birds were “at risk of extinction.”

XR member Joseph Young, 43, attended with community worker Laura Churchill and their two children Jurno, five, and Fox, 10.

“We are here to save the planet from people who destroy it,” said Fox, who was wearing a tiger costume.

Jurno, wearing a cheetah costume, added: “They are my favorite animals, I want them to be protected.”

The march ended in Parliament Square with a mass “die-in,” which the activists described as “a symbolic spectacle” where participants “lie down in silence, in memory and mourning for the heartbreaking 70 percent decline in wild animal populations since the first Earth Day in 1970.”

“As the government continues to fan the flames of the climate and biodiversity crisis it’s clear that only a collective effort can put it out,” said Greenpeace UK’s executive director, Areeba Hamid.

She said the four-day event would “act as the catalyst of a new united fight against the vested interests putting profits over people and the planet.”

XR has in recent years caused huge disruption, hitting roads, airports and other public transport networks with direct action protests against climate change.

But in January it called a temporary halt to its high-profile demonstrations, and instead promised to mobilize huge numbers against what it sees as government inaction against global warming.

“The climate and ecological crisis aren’t something that is going to happen in the future, it is already here,” said XR spokesperson Zoe Cohen.

“It’s time that the government took this seriously and listened to the people here,” she added.

The group hopes that 40,000 to 50,000 people will attend Sunday’s event, which coincides with the London Marathon.

Discussions have been held with race organizers to reduce disruption.