China has urged Japan to immediately and unconditionally free 14 activists over a protest landing on disputed islands in the East China Sea.
Tensions between Tokyo and its neighbours over the islands - which Japan refers to as the Senkaku Islands and China calls the Diaoyu - have been growing for some.
The activists from China, Hong Kong and Macau - some of whom made it shore - were detained after they travelled by boat to the islands. Five were arrested on the islands, and nine others detained on their boat.
China's vice-foreign minister Fu Ying lodged "solemn representations" on the issue during talks with Japanese officials, including Japan's ambassador to Beijing.
He "demanded that Japan ensure the safety of 14 Chinese nationals and immediately and unconditionally release them", the Chinese foreign ministry said on its website.
Japan protested to China's ambassador over the landing, and Prime Minister Yoshihiko Noda told reporters Tokyo would "deal with the incident strictly in line with the law".
China's Xinhua state news agency said Japan had pushed tensions "to a new high".
Chinese media published photographs of the activists planting the country's flag on a rocky shore.
A separate row over rival claims by South Korea and Japan to other islands has also intensified, signalling the region's failure to resolve differences nearly seven decades after Japan's defeat at the end of the Second World War.