BEIJING (AP) ― Old wounds amplified outrage over a burning territorial dispute Tuesday as thousands of Chinese protested Tokyo’s purchase of islands claimed by Beijing and marked the 81st anniversary of a Japanese invasion that China has never forgotten.

China marks every Sept. 18 by blowing sirens to remember a 1931 incident that Japan used as a pretext to invade Manchuria, setting off a brutal occupation of China that ended only at the close of World War II. Demonstrations are not routine, but this year, as Chinese fume over last week’s Japanese purchase of long-contested islands in the East China Sea, they spread across the country.

Outside the Japanese Embassy in Beijing, thousands of protesters shouted patriotic slogans and demanded boycotts of Japanese goods. Some burned Japanese flags and threw apples, water bottles and eggs at the embassy, which was heavily guarded by three layers of paramilitary police and metal barricades.

“We believe we need to declare war on them because the Japanese devils are too evil. Down with little Japan!” said Wang Guoming, a retired soldier and seller of construction materials who said he came to the embassy from Linfen in Shanxi province, 600 kilometers away, to vent his frustration.

In another part of the capital, U.S. Defense Secretary Leon Panetta had a lengthy meeting with China’s national defense minister, Gen. Liang Guanglie, during a three-day trip that U.S. officials have said Panetta will use to press China to seek ways to peacefully resolve its territorial disputes.

Liang told Panetta that China was “resolutely opposed” to the islands’ inclusion in the terms of a U.S.-Japan mutual defense treaty, and hopes the U.S. will honor its commitment to maintain a neutral stance, the official Xinhua News Agency reported.

Protests also took place in Guangzhou, Wenzhou, Shanghai and other Chinese cities. Japan’s Kyodo News agency reported protests in at least 100 cities, and said people threw bricks and rocks at the Japanese Consulate in Shenyang in China’s northeast. However, Shenyang police said by telephone there was no unrest.

China’s authoritarian government rarely allows protests, and the wave of anti-Japanese demonstrations clearly received a degree of official approval.

Many Japanese businesses across China shut their doors as a precaution following recent protests that turned violent and saw the torching and looting of Japanese-invested factories and shops.

The nationalist fervor spread to the Internet, where users of the popular search engine Baidu saw a huge Chinese flag planted on a cartoon image of the contested islands, which China calls the Diaoyus and Japan calls the Senkakus. And all members of China’s elite badminton team, who scored multiple gold medals in the London Olympics, pulled out of a Japanese tournament that began Tuesday.

The islands are tiny rock outcroppings that have been a sore point between China and Japan for decades. Japan has claimed the islands since 1895. The U.S. took jurisdiction after World War II and turned them over to Japan in 1972.

The disagreement escalated last week when the Japanese government said it was purchasing some of the islands from their private owner. Japan considers it an attempt to thwart a potentially more inflammatory move by the governor of Tokyo, who had wanted not only to buy the islands but develop them. But Beijing sees Japan’s purchase as an affront to its claims and its past calls for negotiations.

Beijing has sent patrol ships inside Japanese-claimed waters around the islands, and some state media have urged Chinese to show their patriotism by boycotting Japanese goods and canceling travel to Japan.

Protests since Tokyo’s purchase have been the largest anti-Japanese demonstrations since 2005. They reflect not only China’s strident opposition to surrendering any land it claims, but generations of Chinese anger over Tokyo’s colonial history that periodically bursts to the surface.

In Beijing, streams of people marched past the embassy in orderly groups of about 150 people, herded by police who urged them to remain calm and peaceful. Some toted posters of Chairman Mao Zedong, and many shouted slogans such as: “United, Love China, Never forget our national shame.”

Sun Chao, who works for a Beijing tutoring company, said he was given the day off and came to demonstrate with about a dozen other friends and colleagues. He spent around 150 yuan ($24) on apples and bottled water that he was handing out on the demonstration route, encouraging people to hurl them at the embassy.

“I want to knock down the Japanese national flag,” Sun said.

Japan has seen its own surge of nationalism. Its coast guard said Tuesday that it was questioning two Japanese who landed on one of the islands. Coast guard official Yuji Sakanaka said it was unclear why the two landed.

Members of Japan’s conservative opposition are calling for the government to get tough with China.

“History clearly shows that this is our territory,” Nobuteru Ishihara, a front-runner for the Liberal Democratic Party’s top post, said at a weekend debate with four other contenders. “It’s fundamental that we protect this territory.” He is the son of staunchly nationalistic Tokyo Gov. Shintaro Ishihara, who started the latest round of the dispute by proposing to buy and develop the islands.

A Japanese coast guard vessel issued a warning to a Chinese vessel near the islands early Tuesday. But officials said they could not confirm reports in Chinese state media that more than 1,000 Chinese fishing boats were headed toward the East China Sea island group.

Numerous Japanese factories, shops, restaurants and schools in China were closed Tuesday after some were targeted by looting protesters over the weekend. The China Daily newspaper reported Mazda halted production at its Nanjing factory for four days, Canon closed three factories and gave 20,000 employees two days’ paid vacation, and Fast Retailing shut 19 of its Uniqlo clothing store outlets in China.

The newspaper said more than a dozen Yokado supermarkets and 198 7-Eleven convenience stores under Japanese management were also temporarily shuttered.

Some protesters vented anger at the United States for boosting its military presence in East Asia, a move they say emboldened Japan and other countries to be more assertive in staking rights to territory also claimed by China.

“Wherever America goes, there will be turbulence,” said retired Beijing teacher Sui Xueyan. “Their crimes are no less than Japan.”