Police detained 34 suspects, of whom 5 were arrested, following a raid on Wednesday on a Roma camp in the Drosero district of Xanthi, in northern Greece.
Most charges reportedly involved possession of illegal firearms and narcotics.
Police said more raids were under way at other Roma camps across the country.
The Drosero raid took place a week after a 4-year-old girl nicknamed Maria was discovered in a Roma camp in Farsala, in central Greece, during a similar operation.
Reports Wednesday said that Greek and Bulgarian police officers were conducting a joint investigation with regard to locating a Bulgarian Roma believed to be the biological father of the young girl.
A large-scale operation was reportedly underway in the area of Sandanski, in southwestern Bulgaria.