The Sea Shepherd group claims to have intercepted an illegal fishing vessel in the Southern Ocean.
The anti-poaching group said the Nigerian-flagged Thunder was earlier issued with an Interpol Purple Notice following a joint effort by New Zealand, Australia and Norway.
Peter Hammarstedt, captain of Sea Shepherd boat Bob Barker, said he ordered the Thunder to Fremantle, Australia, to report to law enforcement authorities there.
"I have notified the Captain of the Thunder and his crew that they have been placed under citizen's arrest, that they must cease their illegal fishing activities immediately and report to the Australian authorities. Should they ignore this order, I have notified the Thunder that Sea Shepherd has no choice but to directly intervene in order to obstruct their continued illegal activities."
Sea Shepherd said it was currently undertaking Operation Icefish, the first of its campaigns to target toothfish fishing operators in the waters of Antarctica.