One agency quoted a Wanigela member as saying that the man died from loss of blood after being attacked by a drunken group of men at the market.
The area around Port Moresby's Koki market is known as a flashpoint for clans and youth gangs from around Papua New Guinea - a country with more than 700 languages and where people hold strong village and clan loyalties.
The Wanigelas and Goilalas have clashed around Port Moresby for years.
Police at the scene said warning shots were fired, but a media officer at police headquarters said reports of three or four people being hit by police bullets were being investigated.
After the confrontation, senior police officers and Wanigela leaders addressed the crowd and appealed for calm.
a tribal member said: "As soon as we heard, we mobilised and blocked the roads and burned tyres."
He said the Wanigelas wanted the Goilalas to hand over the alleged killer and pay compensation.