"It (talks) about parents' consent for the marriage. Where does it speak about the girl's interest?"

The Bangladeshi government has defended the legislation, saying it doesn't encourage rape and calling for trust in the local courts to judge the special circumstances.

Half of girls married by 18

Human Rights Watch has slammed the move, calling it a "devastating step backward for the fight against child marriage."

The organization says that Bangladesh has the highest rate of child marriage in Asia.

Of those, 18% are married by the time they turn 15, one of the highest rates in the world.

"The focus now must be on containing the damage caused by Bangladesh legalizing child marriage," said Heather Barr, senior researcher on women's rights at Human Rights Watch.

"Nothing can change the fact that this is a destructive law. But carefully drafted regulations can mitigate some of the harm to girls."

Responding to the international criticism, Bangladeshi lawmaker Rebecca Momin, who chairs the parliamentary Children and Women Affairs' Committee, said the new legislation had been misunderstood.

"Those who are opposing the idea are not aware of the grass-roots society of Bangladesh. A child born from unwanted pregnancy requires an identity in this society," she said.

"Besides, such (special circumstances) will be monitored and probed by the court and local administration. I'm sure they will have the best judgment before providing permission for marriage."

Barr said that girls at risk of unwanted pregnancy face real problems in Bangladesh but child marriage wasn't the answer.

"The answer includes ensuring that schools provide the information adolescents need about sex, and that the health system offers reliable access to contraception and other health services for unmarried young people."

Child marriage has 'lifelong impact'

The law still requires the approval of Bangladesh's President, but that is largely a formality.

"Marrying as a child has a lifelong impact on a person's well-being," UNICEF Bangladesh representative Edouard Beigbeder told CNN. "It limits opportunities and the chance to be a child.

"Child marriage is forced marriage, given that one or both parties are not sufficiently mature to give their full, free and informed consent."

Khushi Kabir, spokeswoman for Bangladesh women's rights group Nijera Kori, told CNN marrying a girl off to her rapist would do nothing to guarantee her a better life.

"If anything, her husband will use and abuse her further and throw her out. This will increase the stigma and destroy her life," she said.

But Beigbeder added while UNICEF was concerned about the new law, progress was being made against child marriage in Bangladesh.

"In early 2000, the child marriage rate for those who were married under 18 was 68%. ... In 2013, the child marriage rate was down to 52%," he said.