Bulgarian authorities have opened an investigation after a video showing machete-wielding vigilantes handcuffing a group of migrants was uploaded online. The amateur footage was recorded in the mountainous area of Strandja, off the border with Turkey, according to local media.

It shows three men being forcibly held to the ground while several vigilantes, some wearing camouflage gear and at least one holding a machete, use black zip ties to secure their hands behind their backs. "No Bulgaria. Go back [to] Turkey," one of the self-appointed border guards tells the migrants in broken English.

The clip was aired by many Bulgarian news channels and prompted the launch of a probe, as prosecutors say it might depict a case of unlawful detention – a crime punishable by up to six years in jail. Border police chief Antonio Angelov told local broadcaster BTV the migrants were Afghan nationals and described their detention as illegal.

Interior Minister Rumyana Bachvarova condemned the incident on Facebook. "We have to pay attention not only to those illegally crossing the borders, but also to those willing to abuse them, for money or cheap fame," she wrote.

However authorities had previously praised volunteer squads patrolling the country's frontier with Greece and Turkey, suggesting help was welcome as long as the vigilante groups did not restrain illegal crossers, but simply alerted police of their presence.

Earlier in April, Angelov said that a group of nationalists that stopped 23 migrants off the Turkish border had acted "very appropriately". "I want to encourage them," he was quoted as saying by the Balkan Insight news website. "This is very risky. It is not a child's game."

Public opinion is also polarised on the so-called "migrant hunters". Dinko Valev, a 29-year-old semi-professional wrestler, become an overnight celebrity after his migrant patrols on quad bike were featured on a national television report that described him as a "superhero", according to the BBC.

More than 170,000 migrants and refugees have reached Europe from Turkey and North Africa so far in 2016, according to the International Organisation for Migration (IOM). Bulgaria has sealed off about a third of its 269km border with Turkey with a razor-wire fence and beefed up border security in recent months to keep them out.