Mia De Schamphelaere, a Christian Democrat senator whose party opposes the law, accused MPs of ignoring medical and ethical concerns because of a "disturbing ideological stubbornness".
"There is too much haste and too little fundamental debate on a subject that is very literally a matter of life and death."
A fortnight ago, a motion signed by 58 representatives in the Council of Europe's parliamentary assembly accused supporters of the legislation of promoting "the unacceptable belief that a life can be unworthy, which challenges the very basis of civilised society".
In an unprecedented joint statement last year, Belgium's Christians, Muslims and Jews expressed "disappointment and sadness" at the proposal.
Jean-Jacques De Gucht, a Flemish liberal MP and supporter of the law, has dismissed the calls for greater reflection and more time for debate.
"The proposal is too important to delay ... there is no age limit for suffering and we want to provide an answer."
Dr Gerland van Berlaer, a paediatrician at University Hospital Brussels, said the law was so tightly drafted that it would apply only to a handful of teenagers in advanced stages of terminal illnesses.
"We are talking about children that are really at the end of their life."
Euthanasia facts
1432
adults medically killed in Belgium in 2012
3
European countries - Belgium, the Netherlands and Luxembourg - have made euthanasia by doctors legal. The Dutch allow euthanasia to be administered to children
42
Dutch psychiatric patients were helped to die last year, up from 14 in 2012
