Police have detained at least 11 people accused of vandalizing cars and shops in Guangzhou during weekend nationwide protests against Japan's "nationalization" of China's Diaoyu Islands.

The government on Monday called for people to demonstrate in an "orderly and rational" manner, one day ahead of the 81st anniversary of the Sept 18 Incident, which marked the start of Japanese occupation of Northeast China in 1931.

More than 80 Chinese cities have seen residents take to the streets to protest Japan's "purchase" of the islands, according to media.

Some of the protests turned violent.

Japan's consulate general in Guangzhou on Monday said some Japanese received threatening phone calls; others were refused by taxi drivers, or kicked, hit or verbally attacked.

On Monday, police in Guangzhou said they have detained seven suspects for damaging a Japanese-model car in the busy Tianhe district, and another three suspects were accused of breaking shop windows. Another person who damaged billboards was placed under administrative detention.

Police in Qingdao, Shandong province, said they arrested "multiple" suspects who assaulted others and looted during Saturday's protests.

Meanwhile, police in Xi'an, Shaanxi province, and Suzhou, Jiangsu province, asked residents to provide information for them to investigate similar offenses.

They asked people through social media platforms to "be rationally patriotic and to protest in line with the law".

People's Daily Online showed that on Saturday and Sunday nearly 30 accounts managed by government departments from nine major cities in China posted on micro blogs, asking people to act reasonably and warning against violence and crimes.

Hong Lei, the spokesman for the Foreign Ministry, said at a news conference on Monday that the government called on the public to "calmly, rationally, legitimately and orderly express their appeals".

"We will also protect the safety of foreign personnel and organizations in China according to the law."

The Diaoyu Islands were first discovered, named and exploited by the Chinese people, and they were under the jurisdiction of China as early as the 14th century.

The Japanese Cabinet finalized "nationalizing" the Diaoyu Islands on Sept 11.

Many Chinese cities have taken measures to step up security as today marks the beginning of the war waged by Japan on China 81 years ago.

In Beijing, the street that the Japanese embassy is on was blocked on Monday. Barriers were erected, and three layers of police with shields guarded the embassy.

More than 1,000 security guards and volunteers sat on stools alongside the street as a helicopter hovered above. Hundreds of police were directing pedestrians, and armed forces could be seen in buses parked nearby.

"We have coordinated with Japanese companies in Beijing about the upcoming memorial day," Wang Wenjie, a publicity official with the Beijing police bureau, told China Daily.

"We asked them to arrange for the day appropriately according to how they could manage things and how effectively they could communicatewith people."

In Xi'an, local authorities announced on local TV on Sunday that rallies and demonstrations must get approval first and are banned in the central part of the city.

Many Japanese-related businesses in China closed on Monday.

Even though protests have been peaceful in Beijing, many 7-Eleven convenience stores — whose parent company is Japan-based Seven & I Holdings Co — closed on Monday, and the flagship store of clothing company Uniqlo took three logos off its building.

Three Jusco-branded shopping malls in Qingdao have been closed since Saturday.

Panasonic has announced its factories in Qingdao will not resume production until today and asked employees to stay at home.

Contact the writer at wanghuazhong@chinadaily.com.cn

Xie Chuanjiao in Qingdao, and Cao Yin and Zhang Yunbi in Beijing contributed to this story.