An investor watches an electronic board showing stock information at a brokerage office. (File photo)

Fifty Chinese brokerages have vowed to pledge a total of 100 billion yuan ($15.7 billion) to backup stock purchases, as regulators move to further prop up the shaking market, reported Securities Times.

China Securities Finance Corp (CSF), the main government rescuer amid the rout, will manage the capital on the behalf and invest in equities or funds underlying the country's blue chip stocks, said the newspaper.

CITIC Securities have authorized to invest 5.4 billion yuan in an agreement with the CSF, which, together with previous commitment, equals to 20 percent of its parent company's net asset by the end of July, it said in a filing to Shanghai Stock Exchange on Wednesday. The brokerages have pledged altogether 21.1 billion yuan to support the market.

Haitong Securities vowed to make a supplemental investment of about 4.5 billion, besides the 15 billion yuan pledged earlier, in order to "maintain the long-term stability of the securities market, promote the positive role of a securities company and fulfill its corporate social responsibility", it said in a statement on Tuesday evening.

The move came after 21 major securities firms first announced on July 4 that they would spend no less than 120 billion yuan, or 15 percent of their total net assets, on exchange traded funds (ETF) tracking the performance of blue-chips.

The government has launched a string of supporting measures, as the market nosedived from a year-long bull. The benchmark Shanghai Composite Index plunged a combined 25 percent in July and August amid concern for economic slowdown.

The Shanghai gauge opened 4.4 percent lower on Wednesday, and traded at 3,070.28, down 3 percent, as of 10:52 am.