BEIJING (AP) – Thousands of anti-Japanese protesters defied government warnings and held demonstrations today in two Chinese cities, amid suggestions that Beijing was trying to use the unrest to block Tokyo’s bid for a permanent U.N. Security Council seat.

The government’s warning apparently stemmed from fears the protests might spin out of control and seriously threaten ties with Tokyo, which have plunged to their lowest point in decades.

In Shanghai, a crowd of about 5,000 marchers headed toward Japan’s consulate. Another protest by some 3,000 people was reported in the eastern city of Hangzhou. In Beijing, hundreds of police were on guard in Tiananmen Square to block a planned demonstration.

As Japan’s foreign minister prepared to fly to Beijing for talks aimed at defusing the tensions, activists posted messages on Web sites calling for a third weekend of protests over Tokyo’s wartime history and the Security Council bid.

Police in Beijing warned protesters could face legal action. A police statement appealed to the public to trust the Communist Party to handle relations with Japan and to avoid doing anything that might damage “social stability.”

Relations between Beijing and Tokyo have plunged to their lowest point in decades prompted by disputes over the Security Council, natural gas resources in disputed seas and new Japanese textbooks that critics say minimize Japan’s wartime offenses.

Beijing is alarmed at a proposal to give Japan a permanent Security Council seat. Such status carries veto power over U.N. actions and is now held by only five governments – China, the United States, Britain, France and Russia.

The protests last weekend appear to have started with grass-roots nationalist groups, but the Chinese leadership probably decided to let them continue in an effort to derail Tokyo’s bid, said Murray Scot Tanner, a China specialist at the Rand Corp. in Washington.

“I think that permitting the demonstrations provides leverage by creating a very public symbol of the depth of anger among the Chinese people toward Japan,” Tanner said.

Japanese commentators had similar assessments.

“Obviously, Beijing is seeking to take advantage of the so-called history issue and the anti-Japan protest as a means of discouraging Tokyo from campaigning to gain permanent membership in the council,” said the newspaper Yomiuri this week.

Premier Wen Jiabao cited the protests Wednesday when he said during a visit to India that Tokyo wasn’t ready for a Security Council seat until it faced up to its history of aggression.

On Friday, a senior Chinese official denied the government supported “extreme activities” by anti-Japanese protesters.

“Such allegations are totally groundless,” State Councilor Tang Jiaxuan told the president of Japan’s Kyodo News agency, according to the official Xinhua News Agency.

However, Tang also was quoted as saying, “The Chinese people really can’t understand how a nation which cannot honestly look at its aggressive history and which cannot correctly understand the feelings of the people of the countries it victimized could be qualified to bid for a permanent seat on the U.N. Security Council.”

In Japan, an envelope containing a razor blade and a note demanding that China stop anti-Japanese protests was sent to a Chinese consulate in Osaka, police said. They said vandals dabbed red paint on the residence of the Chinese ambassador to Tokyo.

Protests were also planned Sunday in the southern city of Guangzhou and elsewhere, according to notes posted on Web sites by Chinese activists.

An anti-Japanese protest also was planned Saturday by a nationalist group in Hong Kong, the former British territory that was returned to Chinese rule in 1997. There was no indication how many people might attend, but most Hong Kong demonstrations are small, brief and peaceful.

The United States and Japan issued warnings to their citizens in China about possible danger. The U.S. Embassy said its staff was told to avoid all crowds until early May.

Japanese Foreign Minister Nobutaka Machimura was to visit Beijing on Sunday and said he hoped to defuse tensions.

“I expect the upcoming meeting will be a step toward creating a better environment,” Machimura said in Tokyo.

The tensions are fueled by both lingering Chinese anger over Japan’s wartime aggression and anxieties at Tokyo’s new military and diplomatic ambitions, as well as competition for energy.

But despite their rivalry, Beijing is eager to preserve important economic relations with Japan, which has some $280 million invested in the Chinese mainland.

The communist government, having abandoned Marxist ideology for capitalist-style economic reform, has resorted increasingly to promoting ethnic Chinese nationalism to bind the nation together.

But leaders who allowed the latest protests could see them backfire should the public assume that future demonstrations will be allowed, setting up a possible clash with security forces, Tanner said.

“An absolute nightmare for the Chinese government is a photograph of an anti-Japanese protester with blood on him from having been beaten by a Chinese cop,” he said.