TOKYO - AS MANY as 300 Internet warnings of mass murder and other death threats have been posted online in Japan after a knifing rampage in Tokyo left seven people dead, media said on Saturday.

Tokyo police said 200-300 death threats appeared on Internet message boards since 25-year-old Tomohiro Kato went on a killing spree on June 8 in the popular Akihabara district, Kyodo News and the Sankei Shimbun said.

Kato, who rammed a truck into a crowd of pedestrians and randomly slashed shoppers with a knife, had posted Internet messages about his anger and loneliness, and foretold of his deadly plans.

While most postings appeared to be pranks, police have so far arrested 20 people on charges of extortion and public nuisance, the press reports said quoting police.

Authorities identified and arrested a 29-year-old man in Shizuoka prefecture after he posted online: 'I'm going to Ikebukuro (in Tokyo) now and will kill 100 people' just one day after the massacre, media said.

A Tokyo Metropolitan Police spokesman was unable to confirm the figures, saying police do not keep a daily tally of web postings. But he confirmed that the 20 arrests were made nation-wide and were not exclusive to Tokyo.

Tokyo police earlier this week sent a 16-year-old girl to a family court for posting messages on the Internet threatening to kill people in Tokyo's Shibuya district, where young people flock.

'I'll kill them all and I'll die too,' the girl said in one message sent from a cellphone to a bulletin board, as quoted by a police spokesman.

'I adore suspect Kato,' she wrote, adding: 'You're so cool, Mr Kato.'

The girl, whose name was withheld as she is a minor, posted a total of 28 messages two days after the Akihabara killing, according to police. -- AFP