Bridges in London were forced to close Saturday because of protests against climate change organized by a group called Extinction Rebellion, city transport agency Transport for London (TfL) said on Twitter.

The group have occupied five bridges across the English capital -- Westminster, Lambeth, Waterloo, Blackfriars and Southwark Bridge -- on what they call "Rebellion Day" to protest against the British government's "criminal inaction" on climate change.

"Extinction Rebellion demands that the UK government immediately tells the truth and declares a climate and ecological emergency, that it reverses inconsistent policies and reduces to net zero greenhouse gas emissions by 2025, and creates a citizen's assembly to oversee these changes," the group said on Saturday.

London Mayor Sadiq Khan "fully respects" the right to protest peacefully, but it "must be within the boundaries of law," according to his spokesman.

Khan's spokesman agreed with protesters that action on climate change is "urgently" required, adding it was "high time ministers stopped dragging their feet" over the "vitally important" issue.

The UK is a "world leader" in climate change, according to a spokeswoman for the Department for Business, Energy & Industrial Strategy (BEIS), while adding "it's clear that all parts of society need to do more."

"We were the first country to introduce long-term climate targets under the trailblazing Climate Change Act, which has helped the country cut carbon emissions by over 40% since 1990 and encouraged other nations to follow our lead," she added.

Downing Street did not immediately respond to CNN's request for a comment on the protests. (CNN)

•Demonstrators take part in a pro-environment protest as they block Westminster Bridge, near parliament.

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