A bill that bans child marriage in Sierra Leone has been signed into law, President Julius Maada Bio said late Tuesday, in an effort to protect girls in the West African nation where about a third are married before adulthood.

The law is being celebrated widely. It criminalizes marrying any girl who is under 18 years old. Offenders face up to 15 years in prison or a fine of around $4,000 or both. Witnesses to such marriages will also face jail or a fine.

"I have always believed that the future of Sierra Leone is female," Bio said on social media. "This and future generations of girls must thrive in Sierra Leone in which they're protected, equal and empowered."

Sierra Leone is home to 800,000 child brides, with half of them married before age 15, according to the U.N. children's agency.

First Lady Fatima Bio was among the key champions of the law that also provides improved access to education and support services for children affected by child marriage.

When it was passed by parliament as a bipartisan bill in June, she called it "a significant step forward in protecting the rights of our next generation."

Human Rights Watch called the legislation a milestone and said it sets a path forward for other countries like Tanzania and Zambia to reverse laws that allow child marriage.

The government should now raise awareness nationwide about the law and address other harmful practices such as female genital mutilation, says Betty Kabari, a researcher in the Women's Rights Division at Human Rights Watch. It should provide support services for married children and children at risk of child marriage, and focus on keeping girls in school.