Trading on Nasdaq was abruptly halted Thursday because of an unexplained technical issue, paralysing the market for thousands of securities and raising new questions about the robustness of US trading systems following a series of high-profile glitches.

The outage saw a large chunk of the US stock market effectively come to a standstill at midday, freezing prices in stocks, exchange-traded funds and options listed on Nasdaq and prompting other trading venues to stop trading those securities. Nasdaq-listed stocks represent about 28% of all shares traded so far this month, according to Bats Global Markets.