Global markets were in tumult on Thursday after attempts by Chinese authorities to support share prices and the currency raised fresh questions about their ability to manage a slowdown in the world’s second-largest economy. The Shanghai stock market was frozen for the second time in four days — after just 30 minutes of trading — when the size of losses triggered “circuit-breakers” designed to prevent panic selling. The renewed sell-off came after the offshore renminbi fell to the lowest level against the dollar since it was established in 2010. Investors were braced for a test of true market sentiment when it opens this morning after the Chinese regulator announced late on Thursday local time it had suspended the circuit breakers altogether. Stock markets and commodity prices dropped around the globe, while leading government bonds and “haven” currencies rallied in a replay of the August market turmoil prompted by an unexpected currency devaluation.