What their attackers bet on is shame: in a conservative society, the horror of an intimate photo leaking, or rumours being spread, is enough to buy a teenage girl’s silence.

“We didn’t even know anyone could harm us through the internet,” said Sharifa Oishee, a schoolgirl in Dhaka, Bangladesh’s capital.

But research shows otherwise: teenagers are overwhelmingly the victims of an alarming rise in cybercrimes in Bangladesh, driven, as elsewhere, by the proliferation of smartphones and social media.

Police last month began training thousands of teenage girls to defend themselves against blackmail or harassment online. Girls and women make up 70% of the victims of cyber-harassment, according to authorities.

More than 10,000 girls have taken part in the workshops during April and May, with Facebook safety a key focus. Dhaka is the second most active city in the world for users of the social network, and abuse on its platform is the focus of more than 60% of all online harassment complaints to police.

Mishuk Chakma, of Dhaka police’s cybersecurity and crime division, said: “Sometimes the criminals superimpose faces of the girls … on to the bodies of nude models or adult films stars to blackmail and defame them.

Facebook Twitter Pinterest More than 500 students from four schoolsa at a cybersecurity awareness camp in Dhaka. Photograph: Shoriful Islam

“Recently we arrested some hackers from Naogaon district [who had] hacked the Facebook IDs of some girls by phishing. Later, the men demanded money from the girls to return their IDs to them.”

Oishee, 15, said the training had taught her how to scrutinise people before accepting their invitations on the social network. “They taught us how to identify fake Facebook accounts and keep a distance from them,” she said. “We also learned not to disclose too much personal information.”

Harder to stop are revenge porn-style attacks. “These photos and videos often trigger troubles in the lives of the girls when they get into new relationships or get married,” said Chakma. “In such situations, marriages are getting into trouble and, in a few cases, the girls are taking extreme steps, like attempting suicide.”

Harunur Rashid, from the Bangladesh government’s IT ministry, said the training did more than just impart skills. “We found that most of the girls and women did not know how to seek help after they become victims of crimes. Some even remained silent and eventually were blackmailed by criminals,” he said. “The classes are a way of telling girls they can talk about these crimes, that they aren’t alone and that police can help them.”

In schools in the Indian capital, Delhi, similar training is under way for Facebook and WhatsApp. Google is reportedly in talks to train children in other states too. Social media safety programmes are also gradually being integrated into school curriculums in the US, Australia and UK.

But the scale of the programme, and its urgency, is particularly great in Bangladesh, which has experienced double-digit growth in internet use each year since 2002. In a country where sex education is still rare, vast numbers of young Bangladeshis are being newly exposed to the full glare of social media, with less protection than their counterparts overseas.

Facebook Twitter Pinterest Students at a community centre. Internet use in Bangladesh has grown by double figures each year since 2002. Photograph: Charles Sturge/Alamy

Another student, Purbasha Sanchary, 18, said online blackmail was becoming common among the girls she knew. “I am sure that many cases are going unreported,” she said. “To avoid stigma and embarrassment, many simply don’t want to report their cases to police or other agencies.”

In 2013, Bangladesh set up a cyber tribunal to try online crimes, especially those with a communal dimension, that might spark religious or political violence. About 450 cases have been heard by the tribunal, but police concede that it lacks manpower, and cases too often get clogged in the system.

Tureen Afroz, a supreme court barrister, said laws also needed to be updated to reflect new, digital crime scenes.

“It’s good that the government is trying to educate girls and women and raise awareness among them,” she said. “But we are still unable to make the best use of smart, electronic evidence to pin down cybercriminals in a court of law. The government should work to update the Evidence Act.”

Until that happens, police hope that greater awareness among girls will help them to resist online predators. “Now I won’t accept a friend request from anyone unless I’m sure of their identity,” Oishee said.

Shaikh Azizur Rahman contributed to this report