A revised anti-stalking law took effect Tuesday to crack down on stalking via social networking services, which counselors warn is enmeshing a growing number of teenagers.

Acts deemed illegal under the law include sending messages repeatedly through social media when the recipient does not want to receive them and relentlessly leaving comments on someone's blog.

Concerning stalking in general, the amended law allows prosecutors to indict a suspect if the person being stalked does not file a criminal complaint for fear of retaliation and extends the maximum prison term from six months to one year.

According to the National Web Counseling Association, there were 10 cases of online stalking in 2012, the year it began tallying comparable data. The number grew to 97 in 2013, while in 2016 it had jumped to 577 as of October.

Many of the people who consult the association have problems with potential dating partners they got to know through social media, and most online stalking victims are teenagers, the association said.

Lawmakers revised the anti-stalking law in the wake of an incident last May in which Mayu Tomita, an idol singer, was stabbed by a male fan during an event in Koganei, west Tokyo.

Last month, the 21-year-old Tomita complained about the tepid response she received after reporting to the police that she was getting social media messages several times a day threatening to kill her.