Chinese regulator vows share support after markets tumble 8.5 pct in a day

(Repeats story issued late on Monday)

* Main indexes plunge more than 8 percent

* China's securities regulator says ready to keep buying

* Spectre of a full-blown market crash revived

* More than 1,500 stocks dive by 10 pct daily limit

* State-owned margin lender repaid loans early - sources

By Samuel Shen and Pete Sweeney

SHANGHAI, July 27 (Reuters) - China said on Monday it was prepared to buy shares to stabilise the stock market and avert "systemic risks", after major indices plunged more than 8 percent in the biggest one-day fall since 2007.

The securities regulator also said market authorities would deal severely with anyone engaged in the "malicious shorting of stocks", in Beijing's latest attempt to stave off a full-blown market crash.

Monday's slump, amid growing doubts about the strength of the world's second biggest economy, shattered three weeks of relative calm as a barrage of support measures helped stabilise values following a sharp sell-off that started in mid-June.

"The lesson from China's last equity bubble is that, once sentiment has soured, policy interventions aimed at shoring up prices have only a short-lived effect," wrote Capital Economics analysts in a research note reacting to the slide.

The CSI300 index of the largest listed companies in Shanghai and Shenzhen tumbled 8.6 percent to 3,818.73 points, while the Shanghai Composite Index lost 8.5 percent to 3,725.56 points.

China's market gyrations have stoked fears among global investors about the broader health of the Chinese economy, hitting prices of growth-sensitive commodities such as copper, which fell on Monday to not far from a 6-year low.

But, while recent stock market weakness will have caught out many retail investors and companies who jumped in as stocks more than doubled in a year, the relatively low rate of stock ownership by households and a disconnect between valuations and economic fundamentals mean the impact on the economy is likely to be less than in other markets.

FUTURES TUMBLE

Stocks fell across the board on Monday, with 2,247 companies falling, leaving only 77 gainers.

More than 1,500 shares listed in Shanghai and Shenzhen dived by their 10 percent daily limit, led by index heavyweights including China Unicom, Bank of Communications and PetroChina.

All traded index futures contracts also fell by their maximum 10 percent limit, with the exception of a few tracking the large cap SSE50 index, which declined around 9 percent.

Some analysts said talk had circulated among traders that the China Securities Financial Corporation (CSFC) had returned ahead of schedule some of the loans it took to stabilise the stock market, highlighting investor concern that Beijing's commitment to supporting prices may be flagging.