Police investigate the scene of a mass shooting in Thailand in which an armed robber killed three people and injured at least five others.

The masked gunman burst into the Robinson shopping mall in Lopburi 150km north or Bangkok just after 9pm local time yesterday (January 9).

He shot dead a security guard followed by a young woman working at gold store before a two-year-old boy was also shot.

The two staff died at the scene and the boy died later at hospital. At least five others - including one tourist - were seriously injured and rushed to hospital.

Police launched a manhunt and today (Jan 10) and offered a 100,000 Baht (3,300USD) reward for information leading to his capture. They said the killed was an ''expert in firearms'' and believe he had trained using the gun previously.

CCTV shows the gunaman - using a large silencer - leap onto the glass counter of a gold store.

He opened fire and was seen fleeing on a motorcycle with valuables before police arrived on the scene.

Pictures from the mall shows large numbers of people wounded in pools of blood.

Police Colonel Kritsana Pattanacharoen said the police know the man identity and searching for him.

He said: "The police are now patrolling all areas and are looking for the man. Witnessed have described his appearance and we know his identity.

"Moreover, we sent the force to the scene inspecting evidence including the fingerprints.

"We will take this extremely seriously and if the gunman resists the arrest the police will shoot him without hesitation.

''We believe the robber has been trained in the use of weapons before this.

''Studying the gestures he made when shooting, it is unlikely to be someone who has never used guns before.

''The equipment he had was specialised and not something that is easily acquired. The search will continue in the morning.''