The four men and two women, aged 21 to 35, were arrested by border police at the southern border crossing of Calafat and deported to Bulgaria in the afternoon, a police spokesman said.
Police "found CDs, magazines, posters and badges with anti-NATO and anti-globalization contents" in a search of the groups' two cars.
Officials said Romania - an EU member since last year - may expel EU nationals if they "threaten public order or national security."
But human rights groups, including the Romanian branch of the Helsinki Committee for Human Rights, criticized the decision to deport the six, saying Romania was "violating fundamental rights of European nationals."
Romania, which will host the NATO summit from April 2-4, has imposed stringent security measures tightening border controls and mobilizing a total 23,000 police officers, firefighters and special police forces.
Foreign Minister Adrian Cioroianu said Thursday that Romania and its NATO allies "will monitor anti-NATO or anti-globalization groups, well before they enter Romanian territory."