At least five people have been arrested after Extinction Rebellion protesters blocked a road near the Houses of Parliament.
More than 1,000 activists are marching through central London to kick off 10 days of protests in a bid to highlight the climate change crisis.
Four marches converged on Parliament Square after the environmental group planned rallies to go ahead at several landmarks in the capital.
Some campaigners sat in the middle of the road next to Parliament Square, which forced traffic to stop on Tuesday afternoon.
It prompted dozens of police officers to take them away to vans parked nearby, as the rest of the group danced, cheered and chanted.
Protesters are also moving through Westminster and Trafalgar Square.
The Metropolitan Police had already imposed conditions against protests planned for Tuesday and said gatherings could only take place off the main roads at Parliament Square Gardens between 8am and 7pm.
Extinction Rebellion co-founder Dr Gail Bradbrook told Sky News the public had “got the message” over the urgency of fighting climate change and apologised for any disruption.
Various stunts are due to take place throughout the protests from today until 10 September, including a “theatrical” display highlighting “how the climate crisis is causing death” and a silent protest outside Buckingham Palace.
Over the weekend, Extinction Rebellion protesters staged demonstrations across England – including at a number of airports to campaign against proposed expansion plans.
Three women were arrested following a protest in Brighton on Saturday, with specialist officers called to escort down demonstrators who had scaled the West Beach cafe on the seafront.
More Extinction Rebellion events to come in London over the next few days include the “carnival of corruption”, which is due to take place outside the Treasury, and the “walk of shame” near the Bank of England.
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Police said no boats, vehicles, trailers or other structures are allowed at the procession at any point on its route on Tuesday.
Met Police commander Jane Connors said: “The reason we have implemented these conditions is that we know these protests may result in serious disruption to local businesses, commuters and our communities and residents, which I will not tolerate.”
She warned that those who fail to comply to the rules may face arrest.
More than 1,700 arrests were made within Extinction Rebellion’s 10-day “Autumn Uprising” last year, which saw major disruption across the UK and large parts of central London blocked off.