Extinction Rebellion protesters on Tuesday marched across London from five different starting points in the city.
They were set to converge on Parliament Square to coincide with UK MPs returning from summer recess.
It kicked off 10 days of demonstrations across central London as part of the movement's ongoing campaign to highlight climate change.
The city's Metropolitan Police stated gatherings on Tuesday could only take place off the main roads at Parliament Square Gardens between 8 am and 7 pm.
But the group planned demonstrations at several landmarks before marching to Parliament Square as well as Trafalgar Square, which lies at the other end of Whitehall, the centre of the UK government.
The Met also prohibited the use of boats, vehicles, trailers or other structures at any point in Tuesday's march.
"Failure to comply with these conditions may result in arrest," the police force said in a statement.
Last weekend, Extinction Rebellion put on protests across England that included action opposing the expansion of a number of airports.
Watch live footage of the march in the above video player.