HONG KONG — Anti-Japanese protests spread across China over the weekend, and the landing of Japanese activists on a disputed island Sunday sharply intensified tensions between the two countries.
Thousands of protesters took to the streets in nearly a dozen Chinese cities Saturday and Sunday in response to Japan’s detention Wednesday and deportation Friday of activists from Hong Kong, Macau, and China who had landed on the island, part of a chain of uninhabited islands known as the Diaoyu in China and the Senkakus in Japan.
Demonstrations took place in cities up and down China’s eastern provinces, according to Xinhua, the official news agency. Photographs posted on Sina Weibo, the country’s most widely used microblogging service, suggested that large numbers of people took part in the protests. In one image said to be from the southwestern city of Chengdu, deep in China’s interior, the number of protesters appeared to be in the tens of thousands. Chinese state media portrayed the demonstrations as fairly small, involving fewer than 200 people, and not extending to inland provinces.
‘‘Defend the Diaoyu Islands to the death,’’ read one banner held at the Chengdu rally. Another photograph showed a handwritten sign taped to the entrance of Suning, a popular electronics store, telling customers it was no longer selling Japanese products.
Some protests appear to have turned violent. According to several postings, demonstrators Sunday attacked sushi restaurants or other businesses perceived to have a Japanese connection. Several photographs said to be from Shenzhen, across the border from Hong Kong, showed what appeared to be damaged or overturned cars — most of them Japanese models — as well as several police vehicles.
The demonstrations appeared to be sanctioned and chaperoned by police, who generally prohibit public protests unless they suit the needs of the Communist Party.
In the past, Beijing has allowed nationalist sentiment to bubble up into street demonstrations, but the authorities usually keep them contained out of concern they might spiral out of control or metastasize into popular antigovernment sentiment.
Even as the protests began unfolding Sunday morning, a group of conservative Japanese activists might have planted the seeds for further anger in China. About 10 of the activists, including local assembly members from Tokyo, swam ashore to the disputed island, Uotori.
While Japan controls the island chain, the Tokyo government restricts access in order to avoid inflaming regional tensions. The 10 who landed Sunday did so without permission, and were later questioned by the Japanese Coast Guard.
The group said they were responding to the pro-China activists’ landing, and they urged Prime Minister Yoshihiko Noda to do more to defend the islands. ‘‘Four days ago there was an illegal landing of Chinese people on the island,’’ Koichi Mukoyama, a lawmaker who was sailed to the island but did not swim ashore, was quoted as saying by the Associated Press. ‘‘We need to solidly reaffirm our own territory.’’
The Chinese Foreign Ministry reacted angrily, after having asked Japan to make sure no activists reached the island.
‘‘Japanese right-wing elements have illegally violated China’s territorial sovereignty,’’ Qin Gang, a spokesman, said in a statement on the ministry’s website. ‘‘Relevant officials from the Foreign Ministry have already made stern representations to the Japanese ambassador, making a strong protest and urging Japan to cease actions that are damaging China’s territorial sovereignty.’’
The Japanese activists were part of a group of conservative members of Parliament and local politicians who arrived at the island on a flotilla of nearly two dozen boats that carried about 150 people.
The Japanese Coast Guard did not release the names of the activists who had made it to shore. Photos of the landing by the Kyodo News Agency showed several men and at least one woman standing in wet street clothes as they displayed a Japanese flag on the island’s rocky shore. In China, Global Times, a nationalist-inflected newspaper owned by People’s Daily, the Communist Party mouthpiece, held an impromptu seminar on the crisis Sunday, with many participants calling for more radical action.