Stock indexes decline on selling pressure

Share prices fell suddenly in the last hour of trading on Thursday, almost wiping out Wednesday's rally and leaving investors in the dark about reasons for the moves.

The Shanghai Composite Index slumped 2.2 percent to 3,705.77 points, erasing an earlier gain of 1.5 percent. Drugmakers and technology companies led declines. A gauge of 100-day price-swings rose to the highest level in six years.

Trading volumes in Shanghai have halved from their peaks in June, while margin debt, which had fueled a world-beating rally for China's stocks, declined to a four-month low.