Protesters with the HS2 Rebellion offshoot climbed the seven-storey Tower Place West building in the City of London, home to the offices of insurance company Marsh.
The business insures subcontractors for the HS2 project and was targeted by activists who argue construction of the high-speed infrastructure project linking the capital with the north of England will cause environmental destruction.
The two men locked themselves onto the first and second storeys of the building, with a banner reading “Marsh Insurance Ditch HS2”.
Police and fire crews surrounded the building, cordoning off the area and bringing in an extraction team with an excavator to try and get the protesters down.
An employee from Marsh reportedly came out to talk to the protesters and had tried to explain the company’s obligation to subcontractors.
The City of London Police tweeted: “We are with #metpoliceuk and #LondonFire engaging with protesters that have climbed a building on #LowerThamesStreet.
“A number of officers are on scene working to keep disruption in the area to a minimum. We will keep you updated with any further developments.”
HS2 Rebellion spokesman Adam, from London, said: “Essentially, we're targeting the profiteers, the moneymakers, the facilitators behind these ecocidal infrastructural projects because without insurers protecting subcontractors, they wouldn't be able to work on this.
”It's all of our money which is disappearing into the pockets of people in the City who we don't usually see or hear from.“
One activist speaking from the scene, who gave her name as ‘Madge’, said: “We are very adamant that HS2 should stop immediately. It is a huge vanity programme, we don’t need it, people aren’t commuting in the same way they used to. And apart from that, it is as environmentally destructive as it could possibly be.”
The action was part of the ongoing Impossible Rebellion, a series of protests by Extinction Rebellion and related groups on environmental issues.
HS2 Rebellion is planning another protest on 9 September, which the group's spokesperson said would be ”all the way up the HS2 line“.
It will come ahead of a debate in Parliament on 13 September to discuss a petition calling for the high-speed rail project to be stopped.
Previous protests organised by groups opposed to HS2 have centred on parts of the route north of London where ancient woodlands have been cleared to make way for the line.
The Independent contacted Marsh for comment.