Belgium, one of the few countries where euthanasia is legal, is expected to abolish age restrictions this week 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 – including from paediatricians – and people have staged noisy street protests, fearing that vulnerable children will be talked into making an irreversible choice.

Backers such as Dr Gerland van Berlaer, a prominent Brussels paediatrician, believe euthanasia is the merciful thing to do. The law will be specific enough that it will apply only 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 request euthanasia.

"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 paediatric 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 12 December to amend the country's 2002 law on euthanasia so that it would apply to minors, but only under certain 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 on Thursday. Passage is widely expected.

King Philippe, Belgium's constitutional head of state, must sign the legislation for it to go into effect. So far, the 53-year-old monarch and father of four has not taken a public position, but spokesman Pierre De Bauw said that was not unusual. "We never give any comment on any piece of legislation being discussed in parliament," De Bauw said on Tuesday.

Though one opinion poll found 75% of Belgians in favour, there has been vocal opposition.

This week, an open letter from 160 Belgian paediatricians argued against the new law, claiming there is no urgent need for it and that modern medicine is capable of soothing the pain of even the sickest children.

The doctors also said there was no objective way of providing that children possess the "discernment" to know what euthanasia means.

Besides Belgium, the only other countries to have legalised euthanasia are the Netherlands and Luxembourg, said Kenneth Chambaere, a sociologist and member of the End-of-Life Care research group at the Free University Brussels and University of Ghent.

In Luxembourg, a patient must be 18. In the Netherlands, children between 12 and 15 may be given euthanasia with their parents' permission, while those who are 16 or 17 must notify their parents beforehand.