The Chinese government is struggling to contain the collapse of a stock-market rally it helped engineer, announcing late Monday that it will step up its purchases of shares to prop up sagging indexes.

Chinese shares suffered their biggest one-day percentage drop in over eight years Monday, wiping out hundreds of billions of dollars of market value and putting an end to a three-week period of stability Beijing had achieved by intervening with stock purchases and other steps to stop the market’s slide.

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