Stefan Nikolov, chief secretary of Bulgaria's agency for Bulgarians abroad, was arrested on March 2 on charges of unlawfully abetting foreign citizens in acquiring Bulgarian citizenship, Bulgarian media said.

Nikolov was reportedly arrested in his office in the building of the Presidency and will be charged with abuse of power, Bulgarian news agency BTA said, quoting unnamed prosecution sources.

The agency is a key stop in the process of acquiring citizenship, because it certifies whether applicants are of Bulgarian descent. Nikolov was allegedly issuing such certificates to Moldovan and Ukrainian citizens who did not meet the citizenship criteria.

Applicants have to submit their requests in person, but prosecutors had found evidence of more than 100 cases in which applicants never entered Bulgaria to do so, mass-circulation daily Trud said, quoting sources familiar with the investigation.

Nikolov was believed to be part of an organised crime group that arranged the issuing of Bulgarian passports for foreigners hailing from countries with sizeable Bulgarian minorities, such as Moldova or Ukraine.

Stefan Hristov, the former head of the Bulgarian Interior Ministry's Migration directorate, who was arrested in February, was believed to be part of the group as well.

Hristov remains under arrest after a court in Sofia denied him bail on February 25. Prosecutors plan to ask judges to keep Nikolov under arrest too, Trud said.

"With his actions, Nikolov endangered national security. The case against him is solid," the daily quoted overseeing prosecutor Chavdar Angelov as saying.