Beijing - They waved flags, clenched their fists and held up posters with messages like "Diaoyu Islands belong to China" and "Swing a big knife at the Japanese devils".
Thousands of Chinese in more than 20 cities across China showed their anger towards Japan's purchase of the disputed isles called Senkaku by the Japanese, as relations between the two East Asian powers remained tense.
In Beijing, hundreds of protesters gathered outside the Japanese embassy, with some pelting bottles, stones and eggs at the compound, as police with batons and shields held back the swelling crowd.
Japanese clothing retailer Uniqlo closed its outlet in a popular shopping and dining area in Beijing for the day, while Japanese restaurants in other parts of the city displayed the Chinese flag prominently to pre-empt trouble.
And more trouble may be on the way. Activists from Hong Kong and Taiwan are planning a voyage to the isles to assert China's sovereignty, while protests are likely to erupt again on Tuesday, the anniversary of the Manchurian Incident on Sept 18, 1931, which paved the way for Japan's control of north-east China.
Sino-Japanese ties have worsened after Japan moved to nationalise the islands by buying them from a private Japanese owner.
Beijing has said it is the rightful owner of the isles, located near resource-rich waters in the East China Sea, and declared Japan's move illegal.
"We will resolutely safeguard our sovereignty over the Diaoyu Islands," said Mr He Guoqiang, the latest member of the Communist Party's elite Politburo Standing Committee to issue a warning to Japan on Friday.
Taiwan also claims the islands, and has also protested against Japan's action.
First annexed by Japan in 1895, the chain of five islands and three barren rocks were occupied by the United States after World War II. When the US returned Okinawa in 1972, the disputed islands, which are near Okinawa, were put under Japanese administration.
Beijing and Tokyo agreed in 1978 to shelve their ownership spat and develop them jointly. But this consensus has been effectively shattered with Tokyo's move to buy the isles, say analysts.
Beijing has fought back by declaring the coordinates of its territorial waters around Diaoyu to boost the legal case for its claims.
On Friday, it sent six maritime surveillance ships into the area, in what Japan called an unprecedented incursion into its waters.
Japan, in turn, has said it plans to revise a law to allow officers to land on the Senkaku isles to carry out arrests.
Japan's Foreign Minister Koichiro Gemba and Defence Minister Satoshi Morimoto cut short a visit to Australia to return to Tokyo yesterday to deal with the spat.
Ho Ai Li