JAKARTA, Indonesia—Indonesian police said they arrested about 1,500 people in the country’s remote eastern region of Papua after demonstrators there pushed to join a multinational body that would underpin their goal of independence.

Activists said the Monday roundup was the largest ever in Indonesia’s democratic era, which began in the late 1990s.

The Indonesian half of New Guinea island—known as Papua—is ethnically distinct from the rest of Indonesia. It was annexed by Indonesia in the 1960s and since then, it has been home to an independence movement. Many Papuans say the takeover was an illegal land grab by the federal government in Jakarta.

On Monday, protesters called on the Melanesian Spearhead Group, a group of states and political parties of the Western Pacific, to grant a coalition of Papuan pro-independence groups full membership in the body. The group was established in 1986 to build regional solidarity among Melanesian entities and today is made up of Fiji, Papua New Guinea, the Solomon Islands, Vanuatu and an independence movement in New Caledonia.

Last year, the group also made Indonesia an associate member and granted the Papuan pro-independence coalition, known as the United Liberation Movement for West Papua, observer status.