Trading on mainland Chinese markets has been halted for the day after shares fell more than 7% for the second time this week.
The "circuit-breaker" rule, designed to stem volatility, was triggered in the first 30 minutes of trading, making it China's shortest trading day on record.
The slump prompted renewed panic on global markets, with European share indexes falling more than 2%.
Investors are nervous after the central bank moved to weaken the yuan.
This indicates that Beijing is looking to boost exports, as China's economy may be slowing more than expected.