HONG KONG — After several weeks of relative calm, tumult returned to China’s stock markets on Monday, casting doubt on the government’s measures to support share prices.

While the volatility follows a spate of weak economic data, it also reflects a broader uncertainty in the market. Investors, who borrowed heavily to buy shares during the boom, are now increasingly unsettled.

The situation is creating wild swings in both directions.

The main Shanghai share index plunged 8.5 percent on Monday, its steepest one-day drop in eight years. Shenzhen’s main index fell 7 percent. The volatility continued on Tuesday morning, with Shanghai stocks opening down 4.4 percent before recovering somewhat.