China's tumbling stock market showed signs of seizing up today, as companies scrambled to escape the falls by having their shares suspended and indexes plunged after the securities regulator warned of "panic sentiment" gripping investors. 

Beijing, which has struggled for more than a week to bend the market to its will, unveiled yet another battery of measures to arrest the sell-off. 

The People's Bank of China said it would step up support to brokerages enlisted to prop up shares.

The CSI300 index of the largest listed companies in Shanghai and Shenzhen closed down 6.8%, while the Shanghai Composite Index dropped 5.9%. 

With nearly half the market on a trading halt and another round of margin calls forcing leveraged investors to dump whatever shares could find a buyer, blue chips that had been supported by stabilisation funds earlier in the week bore the brunt.

Over 30% has been knocked off the value of Chinese shares since the middle of June, and for some global investors the fear that China's market turmoil will destabilise the real economy is now a bigger risk than the crisis in Greece. 

"Tthe ripple effect from the market correction has yet to show up," wrote Bank of America Merrill Lynch analysts in a note. "We expect slower growth, poorer corporate earnings, and a higher risk of a financial crisis." 

Commodities markets reflected growing concerns about the broader health of the world's second largest economy, with copper prices falling to a six-year low, while oil fell towards $56 a barrel, near a three month-low.  

More than 500 China-listed firms announced trading halts on the Shanghai and Shenzhen exchanges today, taking total suspensions to about 1,300 - 45% of the market or roughly $2.4 trillion worth of stock - as companies hurried to sit out the carnage. 

The plunge in China's previously booming stock markets, which had more than doubled in the year to mid-June, is a major headache for President Xi Jinping and China's top leaders, who are already grappling with slowing growth.
Beijing's response has also raised questions about its ability to enact the market liberalisation steps that are a centrepiece of its economic reform agenda. 

China has orchestrated brokerages and fund managers to promise to buy billions of dollars' worth of stocks, helped by a state-backed margin finance company which the central bank pledged today to provide sufficient liquidity. 

The securities regulator said the Securities Finance Corp had provided 260 billion yuan ($41.8 billion) to 21 brokerages, though that sum is only 40% of the amount of leveraged positions that investors have cut since June 18.

Unlike other major stock markets, which are dominated by professional money managers, retail investors account for around 85% of China trade, which exacerbates volatility. 

A surprise interest-rate cut by the central bank at the end of June, relaxations in margin trading and other "stability measures" have done little to calm investors. 

The barrage of official commentary and new support measures continued throughout today's trading session, without visible effect. 

Deng Ge, a spokesman for the China Securities Regulatory Commission, said in remarks posted on its official channel on Weibo, China's version of Twitter, that there had been a big increase in "irrational selling" of stocks. 

Government agencies also announced that insurers would be allowed to by more blue chips and urged major shareholders and top executives to buy their own shares. 

But the market sell-off has extended beyond the mainland, with Chinese stocks on US exchanges falling as much as 6.1% last night, according to the Bank of New York Mellon index of such securities.

Hong Kong's Hang Seng index fell 5.8%, with shares of Chinese brokerages taking a heavy beating.