China's 21 major securities brokerages have vowed to "firmly" stabilize the country's stock market, which is suffering from continued plunges.

The brokerages issued a joint statement on Saturday, saying they will spend no less than 120 billion yuan, or nearly US$ 20 billion, on blue chip-based exchange traded funds. The amount accounts for 15 percent of their total net assets.

The brokers will not sell the stocks they were holding on July 3rd and will buy more stocks while the benchmark Shanghai Composite Index is below 4,500 points.

They will also actively repurchase stocks in their own companies from the market and encourage major stock holders to increase their stakes. China's top three brokers -- CITIC Securities, Haitong Securities and Guotai Junan Securities -- were among the 21 signatories of the statement.

The Securities Association of China said in a statement that it appreciated their decision and asked all brokerage firms to view China's economic situation and capital market in a correct way and take similar actions to underpin the ailing market.