Hong Kong's benchmark stock index fell to its lowest since mid-2013 on Thursday, as panic from China's 7 percent market free-fall spilled over the border. The Hang Seng index dropped 3.1 percent, to 20,333.34, the lowest close since July 2013. The China Enterprises Index tumbled 4.2 percent, to 8,753.97 points, a closing level not seen since October 2011. All main sectors fell, with sentiment soured by the fresh slump in mainland equities. China's benchmark CSI300 index collapsed 7 percent after less than 30 minutes of trading, triggering the circuit breaker mechanism that halted share transactions for the day. The tumble in China was triggered by a further weakening in the yuan, gloomy prospects for China's economy and fears of an equity supply glut when a share-sales ban expires on Friday.