Concern mounted in Greece on Thursday over the failing health of a prominent far-left hitman, whose life is in danger after a long hunger strike in prison.

Dimitris Koufodinas, formerly the top assassin for the defunct November 17 extremist group, was admitted to a hospital in Lamia, central Greece, on Monday in serious condition after refusing food for 48 days.

His wife Angeliki Sotiropoulou told AFP on Thursday that the 63-year-old is currently in "critical condition" and "borderline comatose" after refusing force-feeding.

Koufodinas is seeking a transfer from a high-security prison in Domokos, central Greece, to Korydallos prisin in Athens to be near his family.

Dimitris Koufodinas' autobiography 'Born November 17' sold thousands of copies
Dimitris Koufodinas' autobiography 'Born November 17' sold thousands of copies AFP / LOUISA GOULIAMAKI

Amnesty International's Greek branch also recently noted that Domokos is badly overpopulated.

"In today's Europe, it's unheard of to have a prisoner led to death or irreparable harm for asking to be transferred to another prison," his lawyer Ioanna Kourtovik told AFP.

The authorities' refusal to satisfy his demand is "arbitrary and punitive", she said.

Hundreds of lawyers and intellectuals have signed petitions calling on judicial authorities to respect Koufodinas' rights.

The union of Greek judges and magistrates joined the call on Wednesday, urging the authorities to "reconsider" their stance and take "all legal measures to protect his life and health."

Dubbed 'Poison Hand' by Greek media, Koufodinas was the main hitman of the far-left Greek extremist organisation November 17 that was dismantled in 2002.

He is serving multiple life sentences, having been convicted of 11 killings.

November 17 launched dozens of attacks between 1975 and 2000 that killed 23 people.

They included the CIA station chief in Athens, several prominent Greek businessmen, Turkish diplomats and a British military attache.

One of Koufodinas' victims was Pavlos Bakoyannis, a lawmaker who was the father of Athens Mayor Costas Bakoyannis and brother-in-law to Prime Minister Kyriakos Mitsotakis.

On Tuesday, supporters of Koufodinas demonstrated outside the official residence of Greek President Katerina Sakellaropoulou, calling on her to intervene.

On Thursday, deputy environment minister Giorgos Amyras said unknown assailants had vandalised his office, daubing a message of support to Koufodinas on a wall.

An anarchist icon, Koufodinas in 2014 published an autobiography that sold thousands of copies, to the outrage of his victims' families.