New Democracy supporters were voting in the final round of the party’s leadership election on Sunday.
Evangelos Meimarakis and Kyriakos Mitsotakis are battling it out for the conservative presidency after finishing first and second respectively in the first round of voting in December.
The voting process was reportedly going smoothly apart from at a polling center in the Athens suburb of Nea Philadelphia, were masked men forced their way into the building and destroyed ballot papers. Nobody was hurt.
The anarchist group Rouvikonas (Rubicon) later claimed responsibility for the raid via a statement published on the Internet.
Voting is due to end at 7 p.m. but the final result is not expected until around midnight on Sunday.