A 52-year-old Russian man was headbutted and suffered a bleeding nose during an argument with one of the leaders of an anti-government demonstration in Pattaya on Sunday evening.
The incident occurred shortly after 6pm, when about 30 protesters gathered at Jomtien beach.
The demonstrators called for Prime Minister Prayut Chan-o-cha's resignation, the rewriting of the constitution and reform of the royal institution.
At one point, while Sirawith "Ja New" Serithiwat was speaking, a foreigner approached and began talking to the protest leaders. He and Narathiwat "Kane" Khamma got into a heated argument, and a fight started.
Mr Narathiwat was seen shoving the foreigner with his hands and headbutting the man in his face. The foreigner began bleeding from his nose.
Police and officials on security duty rushed to stop the fight. The Russian was given first-aid by rescue workers. His 10-year-old daughter, who was by his side, was crying.
The demonstrators quickly dispersed.
Mr Narathiwat and the Russian, whose name was reported to be Vladislav, were taken to Pattaya police station for questioning.
