A small explosion outside a police station in Vietnam's southern Ho Chi Minh City injured an officer on Wednesday, state media reported.
The cause of the explosion, near the scene of major protests earlier this month, was not immediately clear.
Images on state media showed the station's charred walls with debris from a destroyed motorbike strewn across the pavement.
State-run VNExpress news site reported that a policewoman was injured in the incident.
A witness told AFP he heard two explosions in quick succession.
"I ran to the scene, I saw smoke, it looked very chaotic... even now I'm still very panicked," said the resident, declining to be named.
Cong An Thanh Pho Ho Chi Minh news site, the official mouthpiece of city police, said other witnesses speculated the explosion "could have been be caused by a motorbike malfunction".
Bomb blasts, even small ones, are very rare in communist Vietnam where dissent is strictly controlled and activists are routinely jailed.
The incident comes just 10 days after large-scale protests in the city against a government proposal to set up special economic zones that would grant investors lengthy leases.
The rare demonstrations, sparked by fears that land would be handed over to China, lit up several cities across Vietnam. At least 30 people are still in custody after the protests, including an American being held in Ho Chi Minh City for "causing public disorder".
Security was tight in the city over the weekend, with uniformed and plainclothes police stationed along busy roads and in public parks.
Though political activists very rarely plot violent attacks in Vietnam, police in December convicted 15 people for terrorism over a failed bomb plot at the country's largest airport in Ho Chi Minh City.
Three police officers were killed in an explosion at a police post in central Dak Lak province in 2016, in what authorities later said was an accident.