MUMBAI - Campaigners are using social media and lobbying for special laws to annul child marriages in India, which has one of the world's highest rates of women married as children.

Kriti Bharti, founder of Saarthi Trust, has helped nullify more than 30 child marriages in the western state of Rajasthan, and recently used Facebook posts as evidence in court in a state where these marriages are often held secretly at night.

"We have a law against child marriage in the country, but that law is not being implemented properly," she told the Thomson Reuters Foundation.

"For the first time in my career, I used evidence from Facebook, and for the first time, I think, the court admitted such evidence and annulled a marriage."

India accounts for a third of the global total of more than 700 million women married as children, according to UNICEF, the United Nations children's agency.

Despite a law banning girls from marrying before they turn 18, the practice is deeply rooted in tradition and widely accepted in Indian society. It is rarely reported as a crime and the police are often reluctant to prosecute offenders.

While boys also marry before the legal age of 21, girls are disproportionately affected.

Early marriage makes it more likely that girls will drop out of school, and campaigners say it also increases risks of sexual violence, domestic abuse and death in childbirth.

In Bharti's recent annulment case, Sushila, who was married at 12, had no photographs or registration to prove her marriage, which was denied by her young husband and his family.

Bharti scoured the boy's Facebook profile and found evidence, including posts congratulating him on his marriage, which the court accepted.

While other activists are using mobile apps and cash incentives to deter child marriage, Bharti believes rescuing underage brides is also critical. urn:newsml:reuters.com:*:nL5N1M03FQ

"Taking action to prevent child marriage is important," she said. "But annulments and voiding of marriages that have already taken place can do more to discourage other families from even thinking about it."

States are also acting to strengthen the law: southern Karnataka state this year amended its child marriage act to make any such marriage automatically invalid.

Earlier this month, the Supreme Court struck down a decades-old clause in the country's rape laws permitting a man to have sex with his wife if she is aged between 15 to 18 - ruling that it was rape, and therefore a criminal offence. urn:newsml:reuters.com:*:nL4N1MM4JT

The landmark verdict will help curb child marriage, campaigners said.

(Reporting by Rina Chandran @rinachandran, Editing by Katy Migiro. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit news.trust.org to see more stories.) ((Rina.Chandran@thomsonreuters.com; +91 22 61807242; Reuters Messaging: rina.chandran.reuters.com@reuters.net))