Chris Packham was among the climate change protesters who have a set up camp outside the headquarters of HS2 in London.
The naturalist was one of around 200 people campaigning against the rail project's destruction of trees to create the high speed line.
Protesters held up banners and placards which read "stop ecocide" and "save our countryside" while others chanted "HS Boo".
Activists brought an array of small trees and saplings as they set up a makeshift woodland near Euston Station in London. Protesters appeared in good spirits, with one pair dressed as a badger and a tree.
Packham said HS2 involves one of the "largest deforestation programmes since the First World War", damaging more than 100 ancient woodlands along the planned route, along with dozens of wildlife habitats.
He told protesters: “If this project progresses pending the review it will damage 108 of our ancient woodlands.
“I see these as natural and national treasures.”
The government has ordered clearances of ancient woodland for HS2 to be stopped while the project is reviewed, unless they are necessary or avoid major costs and delays.
Packham said he fears woodlands are still in jeopardy and that any damage caused to them before a final decision on HS2 amounted to "extraordinary vandalism".
The TV presenter said the government responded to the legal action he launched. He said the Department for Transport has given away responsibility for enacting any enabling works to HS2 Ltd but officials will check over decisions.
Packham said the final HS2 decision will be the first big test for the Government since it declared a climate emergency.
He added: “This is a project that will be damaging to the climate and environment. As a consequence of that this is a potential game-changer.
“It’s very important we try to win this cause and send a clear message we are no longer prepared to put up with things that damage our climate or environment.
“If it wasn’t this it would be the third runway (at Heathrow).”
Transport Secretary Grant Shapps told HS2 Ltd to assess what removals could be halted until after an inquiry into the project was carried out.
All other preparatory works will continue during the review.
Douglas Oakervee - HS2 Ltd's former chairman - is carrying out the review. His final report is expected to be completed in the comings months.
Mr Shapps published a report by HS2 Ltd chairman Allan Cook which warned the final phase of the railway may not open until 2040 and the project could cost £88 billion based on 2019 prices.