The arrests follow the discovery of several headless elephant carcasses in Pelalawan district in Riau province.
"For the first time in Riau's history, we have managed to dismantle the entire network from top to bottom," Ade Kuncoro, director of special criminal investigations at the Riau police, said on Tuesday.
"We have detained 15 suspects, and three others are still on the run," he said.
They could face up to 15 years in prison if found guilty under the country's nature conservation law.
Police said the group operated as a structured syndicate with clearly defined roles.
Those arrested included the shooters, individuals who removed the elephants' heads to extract tusks, suppliers of home-made firearms, financiers and brokers who arranged ivory sales, Ade said.
He linked the group to at least eight elephant killings in 2024 and four more in 2026, all carried out by shooting.
The killings highlight the precarious state of the Sumatran elephant, a subspecies found only on the Indonesian island and listed as critically endangered by conservation groups.
Fewer than 3000 are believed to remain in the wild, according to environmental organisations, with habitat destruction driven by palm-oil plantations and logging compounding the threat from poaching.
Police did not say whether the suspects were connected to international trafficking routes or how much ivory had been seized.
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