Kicking off a summer-long campaign of civil disobedience and grassroots activism, thousands of campaigners with the Extinction Rebellion movement shut down traffic in five U.K. cities on Monday to demand that the government take immediate and sweeping action to combat the climate crisis and ensure a sustainable future.

Campaigners risked arrest by blocking major roads and bridges in Bristol, Cardiff, Glasgow, Leeds, and London with colorful boats named after legendary environmentalists like Polly Higgins, who died of cancer in April.

The boats conveyed a simple message: "Act Now."

As The Guardian reported, "Protests in each city are focusing on a different ecological threat: rising sea levels, floods, wildfires, crop failures, and extreme weather. According to Extinction Rebellion, more than 3,000 activists across the country have signed up to participate in acts of civil disobedience this week, a third of those in London."

"We are facing the sixth mass extinction," said Frances, an 18-year-old Extinction Rebellion activist who took part in Monday's action. "If there was one thing I could say to our government it would be: Act Now. Our lives are in your hands."

In a statement, Extinction Rebellion said it is calling on the British government to act to "halt biodiversity loss and reduce greenhouse gas emissions to net zero by 2025."

"We've all read the science, we know the story, the whole phase of denial is over, and if it takes civil disobedience to make a difference, then so be it," Wilf, a 50-year-old teacher who took to the streets Monday, told The Guardian.

Extinction Rebellion's summer mobilization is part of a global movement pressuring governments to confront the climate emergency with bold and just solutions that match the scale of the crisis.

As Common Dreams reported earlier this month, activists with the global youth-led Fridays for Future movement have continued taking to the streets during their summer break because, as 16-year-old Swedish campaigner Greta Thunberg put it, "the climate crisis doesn't go on summer holiday."

"And neither will we," Thunberg said. "We go on."