Bailiffs from HS2 have started to evict activists who dug a tunnel near Euston station in protest against the £106bn rail project.
It comes after the BBC revealed campaigners spent months digging the tunnel they claim is 100ft (30m) long.
Since August, HS2 Rebellion members have been living in tree houses and tents at a camp nearby.
A HS2 spokeswoman said the protesters were "trespassing" on land owned by the company.
The land being occupied is needed for continued building work around Euston, she added.
Enforcement agents from the National Eviction Team have removed some protesters from the makeshift camp in the park.
Police have arrested five men and a woman at the site, although one male was later de-arrested.
Activists say the tunnel - codenamed "Kelvin" - was dug as their "best defence" against being evicted.
Protesters said they were continuing to dig tunnels and have vowed to stay for as long as possible.
An 18-year-old, who gave his name as Al, said the tunnels can only be accessed through a section of the makeshift camp and were about 15ft (4.5m) deep.
"I will stay as long as I can," he said, but he added the activists "have not got much food and water".
HS2 Rebellion told the BBC four people had "locked themselves" to fixing points inside the tunnels.
One activist, Blue Sandford, admitted the stunt was "dangerous" but felt it was "worth it".
The 18-year-old, who is currently on school strike for climate, said HS2 "is a waste of money".
"I'm in this tunnel because they [the government] are irresponsibly putting my life at risk from the climate and ecological emergency," she said.
"They are behaving in a way that is so reckless and unsafe that I don't feel they are giving us any option but to protest in this way to help save our own lives and the lives of all the people round the world.
"I shouldn't have to do this - I should be in school - the trouble is they are stealing that future and I have to stop them."
Martin Andryjankczyk, who was carried out of the camp by enforcement agents earlier, predicted it would take "at least a week or two" to evict all the protesters.
The 20-year-old was taken to Holloway Police Station when he was led away but said he had been "de-arrested" and returned to the park.
"I have been living here for the last four months. They (the remaining demonstrators) aren't going to give up that easily," he said.
The Met Police confirmed a number of officers were sent to the eviction site at Euston Square Gardens to assist High Court enforcement officers should there be any breach of the peace and to uphold Covid legislation.
The force said five people who were arrested at the site remain in custody.
A spokeswoman for HS2 said tunnel protests were "costly to the taxpayer".
She added: "HS2 has taken legal temporary possession of Euston Square Gardens in order to progress with works necessary for the construction of the new Euston station.
"These protests are a danger to the safety of the protesters, our staff and the general public, and put unnecessary strain on the emergency services during a pandemic."
HS2 is set to link London, Birmingham, Manchester and Leeds. It is hoped the 20-year project will reduce rail passenger overcrowding and help to rebalance the UK's economy.
The campaign group alleges HS2 is the "most expensive, wasteful and destructive project in UK history" and that it is "set to destroy or irreparably damage 108 ancient woodlands and 693 wildlife sites".
However, HS2 bosses have said seven million trees will be planted during phase one of the project and that much ancient woodland will "remain intact".
At the scene
BBC London Home Affairs correspondent Katharine Carpenter
There is a ring of security surrounding the square outside Euston Station and a crowd of journalists reporting on today's event.
Every now and then there is a burst of singing through a loud hailer and motivational speeches echo from the trees.
Most of the protesters we can see are among the branches, some have cut their safety lines, others are swinging in harnesses.
Earlier, enforcement officers were lifted up in a cherry picker into one of the tree camps . They have spoken with the demonstrators and are now fixing ropes to the high level platforms.
We've been told at least four people are inside the tunnels HS2 Rebellion have dug under the site.
People inside the fence have said they predict the eviction to "take weeks".
The atmosphere is calm but the police have begun to push back people watching, reminding them of Covid-19 regulations and asking to see press passes.