China's benchmark Shanghai stock index plunged 6.97 percent at the opening on Wednesday despite government moves to shore up the tumbling market, dealers said.

The Shanghai Composite Index slumped 259.72 points to 3,467.40. The Shenzhen Composite Index, which tracks stocks on China's second exchange, dropped 4.07 percent, or 78.61 points, to 1,854.22.