Police have arrested a 41-year-old man who appeared to be helping at the scene of a road accident, but then stole the car driver's 6-million-baht watch and diamond ring.

Police arrested Pongdet Lewtrakul on Wednesday night. They also took a 6-million-baht Richard Mille watch, a diamond ring and the clothes he wore on the day of the theft as evidence.

Police said on Thursday that Mr Pongdet had arrived on his motorcycle at the scene of an accident where a Mercedes-Benz had hit another motorcycle in Pathum Thani on the night of Sept 13.

The car driver was helping the injured motorcyclist and waiting for emergency responders. Mr Pongdet offered to be a witness for the Mercedes-Benz owner and gave him his phone number.

After the injured motorcyclist was taken to hospital, the car owner found that his bag was missing from the  car. It contained a Richard Mille watch and a diamond ring.

The car owner phoned Mr Pongdet, who denied any knowledge of the bag. He did not answer later calls.

Police said that Mr Pongdet, who worked at a coffee shop in Pathum Thani, confessed to the theft.