HONG KONG — Struggling to respond to precipitous declines on China’s stock markets over the last three weeks, the country’s biggest brokerage firms unveiled a government-endorsed plan on Saturday to buy shares starting on Monday, while both of the country’s stock exchanges suspended all further initial public offerings of stock.

The government-controlled Securities Association of China said that 21 big brokerage firms had agreed to set up a fund worth at least 120 billion renminbi, or $19.4 billion, to buy shares in the largest, most stable companies, and to stop selling shares from their own portfolios. But some experts said the moves might not be enough to stop the hemorrhaging of money from the stock market, particularly given that $105 billion in shares changed hands in Shanghai on Friday.

The association did not specify whether the fund would be allowed to use its initial purchases of shares as collateral to borrow money, probably from the government, and buy more shares. That could give it the leverage to have a bigger effect on the market.

“If it’s only 120 billion renminbi, to be honest, it’s too small,” said Hao Hong, the chief strategist of the Bank of Communications International, the global finance unit of China’s fifth-largest bank. “But if you can add leverage on top of it, that could support the large-cap, blue-chip stocks.”