The protest sparked travel chaos in the capital, as commuters tried to make their way to work.
Pictures from the scene showed protesters dangling a banner over Tower Bridge, overlooking the River Thames. The banner read: "End fossil fuels now."
Extinction Rebellion arrived at the scene at around 7.30am.
A statement on their website reads: "At 7am this morning, Extinction Rebellion UK hung a large 22m x 3m banner from the suspension part of Tower Bridge in London that read ‘END FOSSIL FUELS NOW’.
"Two people are hanging from the bridge by suspension cords and have released red flares. The bridge is currently closed to traffic.
Extinction Rebellion protesters in London
XR
Tower Bridge has been closed off by police
GB News
"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.
"People will gather at Speaker’s Corner every morning for a week or more at the same time, where there will be regular trainings in nonviolent civil disobedience and resistance tactics before marching into London to block areas of the city for as long as possible."
Tower Bridge reopened shortly before 12pm.
Amy Rugg-Easey who was taking part in the action today, 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.
"When I volunteered during the COP26 climate conference in Glasgow last year, I learnt from members of indigenous communities from the Amazon rainforest that companies destroying their home are doing it to extract oil, and some of the indigenous environmental campaigners had been killed while defending their home.
"Some of the banks and companies continuing to fund, insure and endorse these projects are based here in London, where those communities cannot reach them, but I can, so I will do what I can to help.”
Commuters hit out at the "selfish" activists, as central London was hit by disruption during rush hour.
It comes ahead of a "mass participation" protest, starting in London's Hyde Park tomorrow.
Amelia Halls, 23, of Extinction Rebellion, said: “It should come as no surprise that people are choosing to take action like this.
The closure has caused travel disruption in London
GB News
"Scientists are telling us that we cannot continue to invest in fossil fuels if we want to have a livable planet, and are risking their freedom to make the Government listen.
"But rather than listening to the science, the Government’s new energy strategy doubles down on oil and gas extraction in the North Sea.
“While people will struggle to pay their energy bills, energy companies make record profits.
"This opportunism is criminal, and shows disgusting disregard for the needs of ordinary people.
"Well, not in my name. I will not be a bystander any longer, and neither should anyone else who believes in a better world.”