At least 400 drug addicts have escaped a rehabilitation centre in northern Vietnam where they were receiving compulsory treatment.
Local media reported that the group struck down the gates of Gia Minh Centre near Hai Phong city on Sunday.
About 30 eventually returned voluntarily, said AFP news agency. Police are now looking for the rest.
Human rights groups have previously condemned conditions in Vietnam's drug rehabilitation centres.
The BBC's Nga Pham says that in Vietnam, drug addiction is considered a serious social vice.
Drug users are regularly arrested and the government enforces compulsory rehabilitation programmes. Some people are also sent to rehabilitation centres by their families.
Thanh Nien News said that several people vandalised homes as the group of escapees made their way to the city.
A photo carried by the site showed scores of men, many of them only wearing shorts, walking down a street en masse.
A local official was quoted as saying the group had escaped to pressure the authorities for "better policy".
Another told AFP that the treatment period had recently been extended from two years to three, and the centre reduced the amount of money spent on food, prompting complaints.
It is the second such major escape from the centre in a decade, with hundreds of people reportedly leaving in April 2005, reported Thanh Nien.
Detainees at similar centres in Hai Phong also staged breakouts in May 2010 and April 2012.
Reports estimate there are 140,000 to 183,000 drug addicts in Vietnam currently.
Human Rights Watch has condemned conditions in government-run rehabilitation centres, calling them "little more than forced labour camps".
The rights group has claimed that drug users are held without due process for years, forced to work for little or no pay, and suffer torture and physical violence.
But our correspondent says the Vietnamese government has rejected this, saying overall rates of drug use in the country have been reduced thanks to its drug treatment and rehabilitation programme.