Extinction Rebellion protesters shut down Tower Bridge by abseiling off the sides of the London landmark.
Two activists were hanging from the bridge by suspension cords and unfurled a huge banner that read: “End fossil fuels now”.
Police officers cleared the protesters by 11.40am, escorting them into police vehicles, and the bridge was reopened at midday.
"Officers were called to Tower Bridge this morning after reports of protest activity," said Commander Umer Khan, for policing operations and security at the City of London police.
"Tower Bridge was subsequently shut in both directions to traffic and pedestrians and reopened at 12pm. Four people were arrested and they are currently in police custody."
The bridge is a main traffic artery across the Thames, and its closure caused long queues on both approaches.
Tower Bridge in London closed after Extinction Rebellion launches protest
To the north of the river is the financial heart of the City. To the south is London City Hall.
The protesters hung the banner at 7am on Friday and also released red flares.
Extinction Rebellion said: “The action has taken place at the gateway to the City of London – the root source of fossil fuel funding in the UK – and on the eve of the April Rebellion which begins tomorrow at 10am in Hyde Park.”
Amy Rugg-Easey, who is taking part in the demonstration, said: “I ask myself why I do these things all the time, and the main thing that drives me is that I have tremendous hope and optimism in humanity's ability to fight the climate crisis, but there are certain people who continue to prevent that for their own profit.”
The action by Extinction Rebellion, which is calling for an end to all new fossil fuel infrastructure, comes after eight days of disruption at oil depots by the group and the Just Stop Oil coalition.
Extinction Rebellion is known for picking landmark buildings for its protests.
Ben Hersh, 24, an office worker who regularly uses the bridge to get to work, said the protesters should "grow up" and "stop taking aim at regular people".
"I know that climate issues are important, but why are we always targeted?" he said
"They need to grow up, stop doing stupid stunts, stop taking aim at regular people and maybe more people would do things to help them and the environment."
An Uber driver, who referred to himself only as Muhamed Q, said: "I am worried about getting no money, or not enough money, and to jobs."
London mayor Sadiq Khan said that the protest was "counterproductive".
