Extinction Rebellion has planned protests in the capital every day until Sunday, 17 April.
The Metropolitan Police said Oxford Street and Regent Street were blocked.
The group said the action was intended to be “highly disruptive” and “designed to disrupt, engage and recruit new rebels”.
The Met said protesters were disrupting traffic on both streets and said it had specialist teams on standby “to respond to any protesters who lock or glue themselves to street furniture or complicated structures”.
Traffic diversions were being put in place.
Extinction Rebellion spokespeople have said they want people to “flood the streets of London” and that they will “create the most roadblocks we ever have with a new action design”.
On Saturday, the crowd sat down across Oxford Circus and the famous streets it connects, chanting “save our planet” and “whose streets? Our streets”.
They carried banners that read “I am here for nature and children”, “navel gazing into disaster” and “life on earth is dying”.
On Friday, two protesters from the climate movement shut down Tower Bridge by abseiling off the sides of the London landmark and unfurling a huge banner that read “end fossil fuels now”.
The bridge, a main traffic artery across the Thames, was closed to vehicles, causing long queues.
London’s Labour mayor Sadiq Khan said the protest was “counter-productive” adding that demonstrators needed to win over public opinion at the same time as putting pressure on the government.
