A Thai royal supporter attacked a schoolgirl because she did not stand for the national anthem.

The poorly student, 15, was waiting for the train after school yesterday afternoon (October 27) when the establishment theme played through speakers.

Tradition states that when the song plays at 8 am and 6 pm every day, people must stop what they're doing and stand. The custom is followed religiously in public places such as train stations or government offices.

However, the little girl was feeling unwell. When the middle-aged woman noticed her sitting, she stormed over the tracks to reach her on the opposite platform in Phra Nakhon Si Ayutthaya province, central Thailand.

CCTV footage shows how the woman in a red t-shirt pinched and pulled the girl's hair before slapping her face.

The student collapsed in tears, complaining that she had been suffering from period pains. Her friends then comforted her as she recovered from the trauma.

Footage taken by an onlooker shows how the woman, a street food vendor, scolded the youngster and accused her of disrespecting the country.

Confused travellers, who were too scared to intervene, looked on before a station worker arrived and the woman was pulled away.

Police Colonel Prawet Srinark, superintendent of the Phra Nakhon Si Ayutthaya police station, said the schoolgirl told officers that she was on her period and felt pain if she stood up.

The policeman said: "We understood that the student was experiencing her menstruation cycle, which is human nature. The street vendor will be charged with assault by threatening a person's serious injury."

Police Colonel Srinark said they had identified the suspect and she would be summoned to the station to be questioned on suspicion of assault.

The woman who slapped the schoolgirl later hander herself in and was questioned. She was later released without being charged while police continue the investigation and interview the family of the schoolgirl again.