Police and eviction squads have gathered in a standoff with HS2 protesters including veteran activist Swampy, who is sitting in a 30ft bamboo structure over a river in an attempt to stop HS2 building a works bridge through the Colne Valley nature reserve. Activists put up the makeshift tower in the River Colne at the weekend to stop workers erecting the bridge and cutting a temporary road through the Denham Country Park woodland. However, on Tuesday morning, construction crews had set the first half of the steel bridge into place, within a few metres of reaching Swampy’s nest in the river, while security personnel lined the water. Thames Valley police said two men had been arrested on suspicion of attempted assault in connection with the protest Swampy – as Daniel Hooper, 47, is best known – said that attempting to complete the bridge with protesters in the way would be “shockingly dangerous”. Speaking to the Guardian before the standoff, he said: “Normally they’d get a cherry picker and a few police climbers to get me down but in a river it’s quite dangerous.” He said that the Denham protest camp, which has occupied parts of the country park for the last six months, was saving dozens of ancient oaks and other trees from destruction. Swampy said he hoped that the action was “at least slowing HS2 down until the government decides to scrap it”. Contractors have been attempting to build an access road to relocate electrical pylons for the planned high-speed railway, and had put in place the concrete foundations of the bridge. Sam Smithson, an environmental protector, said HS2 had declined proposals to take cut through a nearby golf course rather than the woodland. She said: “Every single day we are witnessing the landscape drastically changing, HS2 is felling trees and irreparably destroying nature before our eyes. “This is a non-violent peaceful protest and we will continue to do this until the government takes its own advice by acting now to stop biodiversity loss which means stopping HS2.” HS2 said the work was not impacting ancient woodland, and that alternative access routes – needed to divert power lines crossing the path of a new viaduct – would have been more environmentally damaging. An HS2 spokesperson said: “HS2 has been repeatedly and excessively targeted by political activists. This criminal activity, including illegally trespassing on our construction sites, is a threat to the safety of the public and our workers, and places unnecessary strain on the emergency services during the pandemic.” Work on the first phase of the £100bn high-speed rail line is accelerating at the major works site three miles away at South Heath in Buckinghamshire. Two 170 metre-long tunnel-boring machines are being assembled to start drilling the 10-mile Chiltern tunnels in spring 2021. The two 2,000-tonne machines were manufactured in Germany before being shipped in parts to the UK. The construction site beside the M25 is the size of 80 football pitches, with the tunnel’s south portals already created ready for the machines. HS2 Ltd’s chief executive, Mark Thurston, said: “The launch of our first tunnelling machines will be a defining moment in the history of HS2 – and our work to deliver a high speed railway that will offer a low-carbon alternative for journeys across the UK.” Major civil engineering work started this year, after the government decided in February to press ahead with the high-speed rail network.