Hydrological workers had already observed cracks in the dam, and helicopters were arranged to evacuate all the emergency workers, mostly soldiers with the People's Liberation Army (PLA) and the armed police.
More than 200 armed police officers worked round the clock for four days to drain the lake. Once overflowed, it could threatens some 1 million residents living on the lower reaches.
A man-made spillway started to drain from the lake on Saturday morning and military engineers used recoilless guns, bazookas and dynamites on Sunday and Monday to blast boulders and other obstructions in the channel and speed up the outflow.
As a result of two massive blasts on Monday evening which broke through the "bottleneck" in the spillway, the water outflow speeded up drastically on Tuesday compared with 80 cubic meters on Monday night.
The Tangjiashan quake lake, formed after quake-triggered landslides from the Tangjiashan Mountain, blocked the Tongkou River running through Beichuan County, one of the worst-hit areas in the May 12 quake.