Japanese authorities closed several busy beaches near Tokyo popular among surfers on Friday after a large group of sharks was sighted off the coast.

A police helicopter spotted some 30 sharks swimming near the beaches south of the Japanese capital, including a group of 10 hammerheads, local media reported. Sharks have been sighted along Japan’s Pacific coast in recent days and officials from Chigasaki in Kanagawa prefecture posted a warning on the city’s website on Friday advising beach-goers to stay out of the water.

Nine beaches in Ibaraki, some 60 miles (100 kilometres) north of Tokyo, banned bathing as a precautionary measure after sharks were spotted cruising the shore, followed by four more along the eastern seaboard. Beaches in Japan are at their busiest at this time of year during the summer holidays.

Japan’s long coastline is home to a variety of sharks, but attacks on people are relatively rare. Other beaches across Japan have also been forced to take action after a spate of shark sightings, including one in the southern island of Kagoshima where a 1.5 metre (5 feet) hammerhead was spotted.