The father of Vincent Lambert, a severely brain-damaged patient at the heart of a bitter right-to-die case, has denounced his son’s “murder” after a French hospital began removing him from life support.

“It’s murder in disguise, it’s euthanasia,” 90-year-old Pierre Lambert told journalists at Sebastopol hospital in the north-eastern city of Reims on Sunday, where his son has been in a vegetative state since a car accident in 2008.

The crash left him a quadriplegic with severe brain damage that doctors say is irreversible. But the question of whether to continue keeping him alive artificially has divided his family and the nation.

On Tuesday, doctors began taking him off life support just days after a final ruling by France’s highest appeals court on a case which has taken the warring Lambert family to France and Europe’s top courts over the past six years.

Lambert’s deeply-Catholic parents, Pierre and Vivianne, have both fought passionately, along with two of his siblings, to keep him alive, while his wife Rachel and six other brothers and sisters believe he should have the right to die with dignity.

Both parents were at his bedside on Sunday just days after doctors began removing his water and feeding tubes while ensuring a “profound and continuous sedation”.

Denouncing the court decision as “madness”, the pair believe their son is merely disabled and have fought to have him moved to a specialist treatment unit.

Backing them is a Facebook group called “I support Vincent” which pledged to hold a demonstration in Paris on Monday which will be attended by the parents’ lawyers.

Lambert, who worked as a psychiatric nurse, often said he would never want to be “artificially kept alive”, some of those close to him say, but he never put anything down on paper.

• This article was amended on 10 July 2019 to remove a term which did not comply with the Guardian style guide.