This is the disturbing moment machete-wielding vigilantes tied up three Afghan migrants in a 'citizen's arrest' after they crossed the Turkish border into Bulgaria.

The migrants had their hands bound behind their backs with plastic handcuffs and were filmed lying face down on the ground in the mountainous Strandja area.

At one point their captors can be heard saying 'no Bulgaria' and 'go back to Turkey immediately' in scenes that have sparked condemnation from human rights group.

Footage captured the disturbing moment machete-wielding vigilantes tied up three Afghan migrants in a 'citizen's arrest' after they crossed the Turkish border into Bulgaria

The migrants had their hands bound behind their backs with plastic handcuffs and were filmed lying face down on the ground in the mountainous Strandja area

At one point their captors can be heard saying 'no Bulgaria' and 'go back to Turkey immediately' in scenes that have sparked condemnation from human rights group

Footage shows the vigilantes dressed in military-style clothing and tying the frightened migrants up while another shot shows one of the men carrying what appears to be a machete.

Border police who arrived at the scene found the three men unhurt and with their hands free.

Such arrests have happened increasingly in recent weeks, targeting migrants arriving through the border with Turkey in a move denounced by the Bulgarian branch of the Helsinki Committee (BHC) rights group.

'These images (show) the most brutal citizen's arrest so far in Bulgaria. The prosecution must open a probe immediately,' BHC president Krasimir Kanev said, warning that a failure to act by the authorities would only encourage such acts.

But the authorities' reaction has been ambiguous.

'This is.. illegal,' border police chief Antonio Angelov told private bTV television in comments a week after he handed out certificates of appreciation to another vigilante group for intercepting some 20 migrants near the border with Turkey.

Prime Minister Boyko Borisov also expressed his thanks to the group's leader in a telephone call. 'The state belongs to all of us, whoever wants to help (protecting it) is welcome,' he said on Monday.

Footage shows the vigilantes dressed in military-style clothing and tying the frightened migrants up while another shot shows one of the men carrying what appears to be a machete

'Citizen's arrest': Border police who arrived at the scene found the three men unhurt and with their hands free

But the BHC denounced such rewards as 'unacceptable', warning that Bulgaria risked turning 'into a cradle of Balkan fascism'.

Bulgaria, which has so far remained on the sidelines of Europe's worst migration crisis since the Second World War, is concerned that the closure of the western Balkans migrant route could see increasing numbers of people trying to cross its territory.

To prevent an influx, it has sped up construction of an 82-mile razor-wire fence along the Turkish border and holds regular army and border police training sessions along the frontier in a show of force.