Arrests were made in 1,808 cases of Internet crime, up by six cases from the same period last year and the highest since 2003 when police started compiling similar half-year data.
For child prostitution (in some cases referred to enjo kosai or compensated dating), in which minors meet with adult clients on dating and other websites, the number of cases jumped 62.7 percent to 275.
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"We will promote activity to ban children from using dating websites while pushing ahead with spreading filtering on cellphones," the National Police Agency said in a report.
Young Japanese often surf websites specially designed to be viewed on cellphones.
Online auction fraud remained the leading cyber crime, with 490 cases but the figure was down 33.2 percent from last year.