An American man who allegedly stole beer from a convenience store in Bangkok while high on methamphetamine and later slashed a policeman's neck when he tried to arrest him will be charged with attempted murder.

According to the Metropolitan Police Bureau, Alexander Lopez, 29, showed a box cutter while he snatched cans of beer from a 7-Eleven on Phadung Dao Road in Samphanthawong district about 10am on Saturday. He then fled the store.

Pol Lt Prasarn Somkanay, 58, was on duty at a nearby traffic light booth at Suea Pa intersection on Charoen Krung Road, and he chased the man on his motorbike and tried to arrest him. The American allegedly resisted and slashed the policeman’s neck with his cutter before again fleeing.

Witnesses rushed the injured policeman to the Bangkok Metropolitan Administration General Hospital, where he received timely treatment.

The fleeing suspect exhausted himself and was finally arrested on Chalermkhet 1 Road in Pomprap Sattruphai district about 11.05am on Saturday.

Mr Lopez tested positive for methamphetamine use, according to police who said he was unable to answer police questions and was uncooperative with interrogators.

They planned to resume questioning the suspect in the presence of representatives from the US embassy on Monday.

In addition to attempted murder, Mr Lopez faces charges of resisting arrest, carrying a knife in public without permission or good reason, robbery and theft, according to police.

Police said the American is also accused to stealing a bag and a mobile phone from Thammasat University about 1am on Saturday.

Pol Lt Prasarn Somkanay was slashed in his neck.

Pol Lt Prasarn Somkanay chased the suspect on his motorcycle before trying to arrest him and being cut.

The thief grabs a pack of beer cans at the convenience store on Saturday.