On the eve of Clean Air Day, the Extinction Rebellion group stopped traffic in Camden, a tourist hotspot in the city.
One protester described air pollution levels in London "horrendous", saying that they significantly exceeded the levels recommended by the World Health Organisation.
"We are not going to have a future if people don't start to act now", according to one activist.
Another protester said that "petitions, marches, it does nothing", adding that "non-violent civil disobedience is the only way to bring people's attention to it".
Earlier in the year, the group known for staging direct-action protests against climate change has set up camps at major points in the UK capital including Oxford Circus, Waterloo Bridge and Westminster.