Activists shut down Tower Bridge as they staged a protest against air pollution.
The protesters marched onto the Thames crossing and blocked off the road on Monday afternoon.
They were pictured sitting in the middle of the road during the demonstration that brought traffic to a standstill.
Images emerged online of at least nine demonstrators on the bridge surrounded by police shortly after midday.
TfL confirmed the road was closed in both directions and advised road users to seek alternative routes.
City of London Police later said seven people were arrested for obstruction of a public highway.
They remain in custody.
The campaigners wrote on Twitter: “Stop Killing Londoners have blocked Tower Bridge in today's direct action.”
One witness wrote on the social media site: “Nine people appear to protesting against something (no banners or loud speakers saying what) & are blocking traffic.”
Another witness wrote: "Sit-in protest closes Tower Bridge. Tourists using it as an opportunity for enhanced, car-free selfies."
Others criticised the demonstrators for causing traffic, with one Twitter users saying: "Some kind of sit-down protest on #TowerBridge about pollution, ironically causing a traffic jam which is far more polluting."
Another added: "This protest on Tower Bridge is a bit beside the point, protesting for air pollution by causing hundreds of vehicles to stop, which is causing pollution."
And another said : "How to get your anti air pollution message across to the masses? Sit in front of a hydrogen bus, of course."
The road was cleared by 1.25pm and has been reopened to traffic.
