Police said the Sunday incident began after supporters began rioting to protest their local boxer's loss in a championship match at the Kota Lama Sport Stadium in Papua province, the Jakarta Post reported.
The Post said most of the victims were women, adding the stadium had only two working exits. The report quoted police as saying the losing boxer's supporters threw chairs at the judges and the winner's supporters responded by throwing bottles and broken chairs, which led to a stampede.
The Jakarta Globe, quoting Papua police spokesman Gede Sumerta Jaya, said the clash began about 11 p.m. local time after the referee announced the winner.
The loser's supporters, unhappy over the decision, became agitated, causing a stampede among the 1,500 spectators in which 18 people were trampled, the report said.
"The bodies of the casualties have been brought to Nabire regional hospital," Sumerta told the Globe. He also said another 15 people were injured, some critically.
Police officials said local authorities were speaking to at least 12 witnesses.