BRUSSELS — Belgium, one of the very few countries where euthanasia is legal, is expected to take the unprecedented step this week of abolishing age restrictions on who can ask to be put to death — extending the right to children for the first time.

The legislation appears to have wide support in the largely liberal country. But it has also aroused intense opposition from foes, including a list of pediatricians, as well as from everyday people who have staged noisy street protests, fearing that vulnerable children will be talked into making a final, irreversible choice.

Backers like Dr. Gerland van Berlaer, a prominent Brussels pediatrician, believe it is the merciful thing to do. The law will be specific enough that will only apply to the handful of teenage boys and girls who are in advanced stages of cancer or other terminal illnesses and suffering unbearable pain, he said.

Under current law, they must let nature take its course — or wait until they turn 18 and can ask to be euthanized.

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“We are talking about children that are really at the end of their life. It’s not that they have months or years to go. Their life will end anyway,” said Van Berlaer, chief of clinic in the pediatric critical care unit of University Hospital Brussels. “The question they ask us is: ‘Don’t make me go in a terrible, horrifying way, let me go now while I am still a human being and while I still have my dignity.’ ”

The Belgian Senate voted 50-17 on Dec. 12 to amend the country’s 2002 law on euthanasia so that it would apply to minors, but only under certain additional conditions. Those include parental consent and a requirement that any minor desiring euthanasia demonstrate a “capacity for discernment” to a psychiatrist and psychologist.

The House of Representatives, the other chamber of Parliament, is scheduled to debate on Wednesday whether to agree to the changes, and vote on them Thursday. Passage is widely expected.