Anonymous has reportedly taken down at least 97 key websites in Japan, as part an anti-whaling campaign.
The hacktivists have been targeting a variety of organisations including aquariums, news platforms and government offices since September.
According to the National Policy Agency, these attacks have been intensifying with hackers using a technique known as "distributed denial of service" (DDos) where a site is disabled by overloading it with traffic.
Japanese broadcaster, NHK, reported that police are warning organisations to backup their sites ahead of the Ise-Shima summit in May next year.
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Anonymous' war with Japan's whaling industry has already targeted Japanese PM Shinzo Abe.
Earlier this month, the group claimed to have taken the PM's website down and posted details on Twitter using the hashtag #OpKillingBay.
In March last year Antarctic whaling in Japan was declared illegal by the International Court of Justice in The Hague.
However, the nation's whaling fleet set out in November with the aim of targeting 333 minke whales in the name of scientific research.Suggest a correction