Police said Tuesday they have arrested a South Korean high school student, who follows the international hacktivist group Anonymous, on charges of hacking some thousands of domestic and foreign websites since last year.

The 16-year-old, whose identity was withheld, is suspected of defacing a total of 3,847 websites from 87 countries between April 2015 and April of this year, according to the Seoul Metropolitan Police Agency.

The suspect allegedly put the message "Website got hacked" on the first screen displayed on the sites, police said.

In South Korea, 141 websites, mainly those of small and medium-sized enterprises, have been damaged by the suspect's hacking.

Police said the number could be even greater as the 16-year-old testified that he started hacking three years ago.

Still, he did not steal any private information from the websites as the main purpose was to demonstrate his ability, police said. He even posted a list of websites he hacked on social media, such as Twitter and Facebook.

The student also took part in a hacking operation that Anonymous orchestrated to support Hong Kong's democratic movement in 2014.

He studied hacking on his own by spending time figuring ways to break into systems after school, according to police. He used programs he could get access to via Google.

The suspect's family was not aware of the student’s alleged hacking activities for three years, police said.

"I did not know hacking websites could be a crime," he was quoted by the police as saying.

Police have sent the case to state prosecutors. (Yonhap)