Authorities last week detained more than 500 people in two separate raids on an online gambling ring in the capital Jakarta and a scamming syndicate in Batam, in eastern Riau Islands province.

Those arrested were nationals of Malaysia, Vietnam, Cambodia, Laos and Myanmar, which all enjoy 30-day visa-free stays in Indonesia, as well as China, which is not included in the waiver scheme.

Indonesian immigration boss Hendarsam Marantoko said the illicit activities were cause for “serious concern”.

“Cases of foreigners involved in illegal activities, including those coming from countries granted visa-free entry facilities, give rise to (a need for) evaluation” of policies, he said in a statement.

Investigators have found that many of those allegedly involved in the illegal gambling centre in Jakarta had entered Indonesia using visa waivers or applied for permits on arrival.

In recent weeks, immigration authorities have carried out more than 6,700 “administrative actions” including more than 2,000 deportations and cancellations of stay permits, Hendarsam said.