Thousands of anti-Japanese demonstrators mounted protests in cities across China on Sunday over disputed islands in the East China Sea, a day after an attempt to storm Tokyo's embassy in the capital.

Beijing was infuriated last week when Japan announced it had bought the rocky outcrops and while Chinese authorities often quickly suppress demonstrations, many of Sunday's events took place with police escorting the marchers.

There were reports of violence. Demonstrators in Shenzhen -- some holding a banner calling for a "bloodbath" in Tokyo -- clashed with riot police, who fired tear gas to disperse the crowd, Hong Kong broadcaster Cable TV showed.

It also showed footage of more than 1,000 protesters burning Japanese flags in the nearby southern city of Guangzhou and storming a hotel next to the Japanese consulate.

Japan's Prime Minister Yoshihiko Noda called on China to ensure the safety of Japanese citizens and businesses after widespread protests on Saturday saw attacks on individuals, establishments and Japanese-built cars.

"This situation is a great disappointment and so we are protesting" to China, he told Fuji Television.

The relationship between China and Japan, the world's second and third largest economies, is often strained by their historical rivalry even though they have significant business links.

The row over the islands, which Tokyo administers and calls Senkaku while Beijing claims them and knows them as Diaoyu, has heightened in recent weeks.

Six Chinese ships sailed into waters around the disputed archipelago Friday, with Beijing saying they were there for "law enforcement", prompting Tokyo to summon the Chinese ambassador to protest what it called a territorial incursion.

Pictures posted on Sina Weibo, China's version of Twitter, showed marches in half a dozen cities around the country on Sunday.

In Beijing, thousands of protesters gathered outside the Japanese embassy, carrying posters of Mao Zedong and Japanese flags scrawled with obscenities, throwing beer bottles and golf balls and singing the national anthem.

But large numbers of police escorted the protesters as they marched back and forth past the building, while volunteers wearing red armbands handed out food and water to the demonstrators and a medical team stood by.

In Shanghai, where there were major protests on Saturday, more than 1,000 demonstrators gathered outside the Japanese consulate, one group chanting "Down with little Japan" while waving flags and banners.

Police in the commercial hub blocked off roads using shipping containers and plastic barriers, but guided marchers through police lines to protest in front of the building.

One Weibo user in the southeastern city of Quanzhou contacted by AFP said: "There's no violence, just peaceful marches under police guidance."

Microbloggers questioned whether Sunday's demonstrations were spontaneous.

"Such large-scale uniform banners and dresses cannot be made in one day. Do you really believe it's people-initiated?" wrote a Weibo user named Linglingqi, urging the banner-makers to reveal who covered their costs.

Another user named Afraxafra said: "I feel such a massive demonstration definitely cannot be organised by a small number of average people."

The protests did not feature on regular news bulletins on the state-run China Central Television on Saturday though they appeared on the front page of the English-language China Daily on Sunday.

Some state media urged protesters to refrain from violence. "Smoking city blocks, overturned cars, faces contorted with anger -- these are not the images of a civilised society," the Beijing Youth Daily wrote on Sunday.

It urged those wrecking Japanese products to "stay cool-headed and self-controlled" and "distinguish between breaking the law and showing patriotism".

China National Radio said that 1,000 Chinese fishing boats were preparing to head to the disputed waters this week after the fishing season in the area resumed.

Another flashpoint could be Tuesday's anniversary of the 1931 "Mukden incident" that led to Japan's invasion of Manchuria, which is commemorated every year in China.