Police say someone purporting to be a member of the international hacker group Anonymous has claimed responsibility for a cyberattack on the Metropolitan Police Department's website late Wednesday.

According to police, the site could not be accessed between 7 p.m. and 9:30 p.m. The problem was caused by the transmission of a massive amount of data, which overwhelmed the site's server capacity.

The cyberattack also created problems accessing the websites of the National Police Academy, National Research Institute of Police Science and local police departments, since they are connected to the same server.

It was the third cyberattack on the MPD's website and the first by Anonymous, police said. The last time the website was attacked was July 2011.

The official websites of Tokyo's Haneda airport and Chubu Centrair International Airport in Nagoya were also shut down briefly on Wednesday, possibly also related to cyberattacks as it also involved big volumes of data being transmitted.

Access to the website of Narita International Airport also became unstable between Tuesday night and Wednesday morning for the same reason.

Similar attacks disrupted access to the Narita airport website from Friday night and the health ministry from Monday.

A message posted on a Twitter account claiming to be related to Anonymous hinted it may have been involved in the attacks.