The National Police Agency said cyber attacks by Anonymous have intensified since September, public broadcaster NHK reported.
The group has made online sites of companies and other institutions inaccessible by inundating them with huge volumes of traffic. This tactic is called DDos, or distributed denial of service.
Anonymous has claimed responsibility for at least 97 cyber attacks.
The hackers have targeted a variety of websites including those for aquariums, groups related to dolphin and whale hunting, airports, news organisations and government offices.
Police have been collecting information to identify from where the attacks were launched. They are using more equipment to analyse communications logs.
Authorities have urged website administrators to set up backup sites and take other protective measures.
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