Chinese took to the streets in protest, as Beijing lodged a formal complaint, urging Tokyo to prevent frictions from escalating further.
Of the 10 who visited the island, five were conservative local assembly members.
“The Senkakus are undoubtedly Japanese territory. It is to be expected that Japanese would take that to heart,” said Eiji Kosaka, an assemblyman from Tokyo’s Arakawa district.
China’s Foreign Ministry protested, summoning Japan’s ambassador to voice its complaints.
Vice Foreign Minister Kenichiro Sasae told Mr. Cheng in a phone conversation that the protests in China were “regrettable” and urged Chinese authorities to ensure the safety of tens of thousands of Japanese citizens there, the ministry said.
China’s official Xinhua News Agency reported protests in cities across the country. Demonstrators burned Japanese flags, overturned or smashed Japanese-brand cars and in some places smashed windows of Japan-related businesses.
Days earlier, a group of 14 Hong Kong residents and mainland Chinese traveled by boat to the islands, some swimming ashore. Protesters in Beijing, Hong Kong and other cities praised them as heroes and burned Japanese flags, but Japan arrested the 14 for landing without authorization.
On Friday, Tokyo deported the group, seeking to quiet the regional spat.
But plans for further visits by activists on both sides appear likely to further inflame the territorial tensions.
Taiwanese Foreign Minister Timothy Yang summoned Japan’s de facto ambassador to Taiwan, Sumio Tarui, on Sunday to lodge a protest over the visit by the Japanese activists to the islands, which are about 120 miles off Taiwan’s northeastern coast.