"We are also opposed to suffering, whether physical or moral, and especially the suffering of children," Archbishop Andre Leonard, conference president, said in a joint statement with Christian, Jewish and Muslim leaders.
"But to suggest minors can decide on their own euthanasia is to falsify their power of judgment and their freedom. To suggest persons with dementia can also be euthanized is to deny their dignity and hand them over to the arbitrary judgment of decision-makers."
Some Belgian legislators have proposed extending a 2002 euthanasia law to include children and dementia sufferers. Two senate commissions will draft a bill, which then would be debated in parliament.
The religious leaders said such a bill risked "the growing banalization of a very grave reality," adding that they were "deeply alarmed ... as citizens relying on philosophical arguments, and as believers inheriting our respective religious traditions."
"Instead of supporting a suffering person and gathering persons and forces around to help them, we risk dividing these forces and isolating the suffering person, branding them guilty and condemning them to death," said the Nov. 6 statement.
About 1,200 cases of euthanasia, most involving terminal cancer, were registered in Belgium in 2012. In summer 2012, a mentally ill man serving 20 years for a double murder became Belgium's first prison inmate to be euthanized.
In November 2012, the government announced plans to follow the Dutch in allowing euthanasia for Alzheimer sufferers, as well as for children "if capable of discernment or affected by an incurable illness or suffering."
In an October survey by Barometre Politique, 75 per cent of Belgium's 11 million inhabitants favoured allowing euthanasia for children in an irreversible coma or vegetative state, while 80 per cent supported it for dementia or Alzheimer patients facing "unbearable grief."
However, in their statement, religious leaders said caregivers and medical practitioners would face pressure to accept euthanasia, while freedom of conscience and consent would lack effective safeguards.
The statement said all forms of suffering cause dismay, "but to prescribe euthanasia for vulnerable people radically contradicts their condition as human beings. We cannot enter into a logic which will lead to destruction of society's very foundations."
Catholics nominally make up three-quarters of the Belgian population, although only one in 10 attends church services.