Waterloo Bridge is a major bus route and the protestors from the group Extinction Rebellion delayed the journeys of far more public transport users than the vehicle drivers they claim should switch to more sustainable modes.
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.
The Metropolitan Police said officers would be used from across the service to provide a "proportionate" response to the protests, which balanced the right to a peaceful protest while ensuring disruption to communities was kept to a minimum.
A TfL spokesman said: "The safety of our customers and staff is our number one priority and we're working closely with the police to manage the impact on London's transport network.
This will be a full-scale festival of creative resistance, with people's assemblies, art actions, stage performances, talks, workshops, food and family spaces", said Extinction Rebellion, the group organising the demonstration. They have left us with no other option.
Speaking at a meditation on the eve of the protests Dr Williams said humans had declared war on nature.
Ms Thompson has previously said of the demonstrations: "It is time to stand up and save our home".
Yen Chit Chong, from Extinction Rebellion in London, said: "This is our last best shot at survival". They plan to engage in acts of non-violent civil disobedience against governments in capital cities around the world.
Monday will see people in at least 80 cities in more than 33 countries hold similar demonstrations on the same environmental issues, campaigners said.
Activists gathered at Hyde Park yesterday and were encouraged to camp there overnight before a march towards Piccadilly Circus today. No arrests have been made.
Police have advised Londoners to leave extra time for their journeys because of possible delays caused by the protest.