UPDATE 5-Chinese regulator vows share support after markets tumble 8.5 pct in a day
* 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 (Adds regulatory comments)
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. Continuación...