The 24-year-old French national, whose name was withheld pending notification of relatives, was stabbed in his left chest about 3am in front of the Flapping Duck guesthouse on Lamphu Rd in Sam Sen Nai area of Phra Nakhon district, according to Lt Kanya Premtha, deputy investigation chief at Chana Songkhram Police Station.
A woman who witnessed the attack, Maninee Promnat, 30, told police called to the scene that she knew the slain tourist, who stayed at the guesthouse.
She and her friends were drinking in the bar at the front of the guesthouse. A man identified only as Pae and his friends came into the bar and they sat at a table next to hers.
After the bar closed, she remained there, and then heard a loud altercation nearby. She saw Pae quarrelling with a Frenchman on a bridge over Bang Lamphu canal.
She and other people went to separate the two men, hoping to end the quarrel. Pae left, but returned shortly after carrying a knife and walked towards the Frenchman.
The French victim, who was reading in front of the guesthouse, ran over to intervene, and chaos erupted, she said. He received a severe stab wound and collapsed to the ground. Pae fled.
Guesthouse owner Eskimo Meenin, 35, said the French victim had been staying there for five days. He was a former exchange student, loved Thailand and was a regular visitor to the country.
Pae was a Thai tattoo artist who had just returned from abroad. He regularly travelled between Thailand and Switzerland.
The Frenchman apparently saw Pae urinating into Bang Lamphu canal from the bridge and told him he should not do that, sparking the quarrel that led to the fatal stabbing of his friend.
Pae fled towards Phra Arthit Rd, but was later apprehended by police, Thai media reported.
The suspect was taken to Chana Songkhram Police Station for interrogation and legal action.
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