Greenpeace said it has blockaded Volkswagen’s UK headquarters in protest against diesel emissions.
The environmental organisation launched the stunt at the car brand’s office in Milton Keynes at around 7am on Monday.
A spokeswoman said activists have barricaded entrances to the building and set up a mock clinic offering health advice to staff and members of the public.
It claimed more than 800 Volkswagen staff employed at the site – in roles such as marketing and distribution – are being prevented from entering.
Concerns over emissions of nitrogen oxides have grown since Volkswagen was found in September 2015 to have cheated air pollution tests for 11 million diesel vehicles worldwide, including 1.2 million in the UK.
Greenpeace is demanding that the firm commits to stop producing diesel cars and switch to only producing electric models.
An estimated 40,000 premature deaths a year in the UK are caused by air pollution, which is linked to health problems such as childhood illnesses, heart disease and dementia.
Greenpeace clean air campaigner Mel Evans said: “As the UK’s biggest seller of diesel cars, Volkswagen is complicit in an air pollution crisis that’s filling up emergency departments and GP surgeries.
“Volkswagen sold us a lie about diesel being clean. Its diesel addiction is seriously harming people’s health.
“Volkswagen won’t meet with us and won’t listen. So today we’ve brought the truth about diesel to its doorstep.
“Volkswagen must face up to its responsibility for deadly air pollution and commit to end diesel production now.”
Aarash Saleh, 33, a doctor in respiratory medicine who is among a group of medical professionals at the protest, said: “Diesel pollution is causing horrendous suffering across the UK and storing up a lifetime of troubled health for our kids.
“If you could see it, diesel would be banned tomorrow.”
It is almost 12 months since Greenpeace activists prevented a cargo ship carrying cars – including Volkswagen diesel models – from reaching Sheerness port in Kent by using boats to climb on board and hang from its 89ft (27m) high unloading door in September 2017.