Bulgaria's security agency arrested controversial Roma boss Kiril Rashkov in drug distribution probe, authorities say.

During the operation carried out by the State Agency for National Security (DANS) he lost consciousness and was taken to hospital, but his condition is reported to be stable.

Rashkov, who is also known as Tsar Kiro ("King Kiro") in Bulgaria, was detained Thursday in the city of Plovdiv, and a probe launched into him is to establish his connections with three other Bulgarian citizens arrested earlier this week.

The three were indicted over possession and distribution of amphetamines, with police seizing 2.7 kg of illegal substances in an operation against them.

They were detained during an alleged drug deal, website Dariknews.bg argues.

An ongoing investigation could also prove their involvement in production of amphetamines, but the men are not yet facing charges, according to the district prosecution.

Rashkov is to remain 24 hours in detention while a probe is under way.