SHANGHAI, June 4 -- Chinese mainland key stock index plunged by a record number of points after the government's main securities daily signaled officials won't try to halt a slump that's erased more than US$350 billion of market value in four days.

The Shanghai Composite Index, which tracks the bigger of domestic stock exchanges, slid 8.3 percent to 3,670.40. The Shenzhen Component Index, which covers the smaller one, lost 7.76 percent to 11,468.46.

The speed that stock prices soared by was "extremely unusual" and highlighted "structural bubbles" in the market, the state-owned China Securities Journal wrote in an editorial.

"There's panic selling," said Yan Ji, an investment manager at HSBC Jintrust Fund Management Co. in Shanghai, which manages about US$517 million. "Investors are convinced the government won't do anything to support the market."

China Vanke Co led declines among property developers after a newspaper report said the government will soon announce measures to cool the real estate market, including increasing the supply of land.

Volatile price moves within each trading day reflected the "weak sentiment" among investors and the fact that the rally was "unsustainable," China Securities Journal, which is affiliated to Xinhua news agency, said.

China's increase in stamp duty is a "proper forward-looking adjustment" to avoid greater "systemic risks" in the market and to ensure its healthy development, the paper said.

Concern that the government will further lift taxes on share trading was heightened on June 1, after figures showed the increase in stamp duty failed to deter investors from opening accounts.

More than 420,000 brokerage accounts were set up on May 30, exceeding this quarter's average of about 300,000, official figures show. The number of accounts last week topped 100 million for the first time

Huaneng Power plunged 1.60 yuan (21 US cents) to 11.89 yuan, while Air China slid 1.07 yuan to 9.68 yuan. They've lost 19 percent and 13 percent, respectively, since stamp duty was raised last week.

China Petroleum & Chemical Corp, Asia's biggest oil refiner, dropping 1.52 yuan to 13.65 yuan.

China Vanke, the nation's biggest property developer, retreated 1.82 yuan, or 10 percent, to 16.38 yuan. Poly Real Estate Group Co, China's third-largest developer by market value, dropped by the daily limit, sliding 3.94 yuan to 35.51 yuan.

The government also plans to build more low-cost housing, the Economic Times reported, citing unidentified sources.

China has stepped up measures to curb lending that's fueling a surge in real estate prices, seeking to maintain social stability. The government in February tightened tax rules on property gains, after it earlier raised interest rates and taxes and restricted lending to developers. Average prices in China's 70 largest cities rose 5.4 percent in April from a year earlier, according to the country's top planning body.

The drop in Chinese shares had little effect on the rest of the region today, with benchmarks in Australia, Japan and Hong Kong all posting gains. The Morgan Stanley Capital International Asia-Pacific Index added 0.6 percent to 152.30 as of 4:18pm in Tokyo, extending a two-day, 2 percent rally.