Climate change protesters have blocked Waterloo Bridge, sparking traffic disruption in central London.
Members of campaign group Extinction Rebellion have also parked a boat at Oxford Circus, and blocked Marble Arch as part of a global day of action.
Yen Chit Chong, from Extinction Rebellion in London, said: “This is our last best shot at survival.”
Police have advised people travelling into London to allow extra journey time. No arrests have been made.
Extinction Rebellion said protests would continue throughout the week “escalating the creative disruption across the capital day by day”.
The Met said it had “appropriate policing plans” in place for the demonstrations and officers from across the force would be used “to support the public order operation”.
In November, activists blockaded the UK’s energy department by chaining themselves together on the pavement.