Angry French fishermen have laid siege to the Greenpeace ship Rainbow Warrior II outside the southern port of Marseille in a row over tuna fishing.
About 20 boats surrounded the Greenpeace vessel after it sailed to the area to highlight the alleged over-fishing of blue-fin tuna.
Port officials later ruled it could not enter the harbour because of the risk of public disorder, an AFP report said.
Greenpeace said it would leave the area at 1800 (1600GMT) on Thursday.
The protest disrupted other vessels trying to use the port, including a number of cargo ships and ferries which were unable to set sail as planned.
'Protest to widen'
The French fishermen began their action at about 0900 (0700 GMT) on Wednesday, and threatened to intensify their protests.
"We are not letting up," Mourad Kahoul, the president of the Union of Mediterranean tuna fishermen, told Reuters news agency on Wednesday.
"If tomorrow, the ship hangs around, we will also close the oil terminal at Fos-sur-Mer."
But Greenpeace France spokesman Pierre Ramel accused the fishermen of being in a state of denial about over-fishing.
"All this is a little pathetic. This is a form of censorship organised by a group of industrial fishermen who own very high-performing fleets and huge fishing resources, and they have no intention of allowing any challenges to large-scale, industrial fishing," he told France Inter Radio.
"They're sitting on a diminishing pile of gold, but as long as there's any left they'll go on drawing from it, until it's all gone."
Greenpeace says illegal over-fishing is threatening to wipe out blue-fin tuna stocks in the Mediterranean.
It is campaigning for an immediate ban on fishing to allow stocks to recover, and had wanted to enter Marseille to put its case.
The fishermen blame other countries for illegal fishing and say a ban would destroy their livelihoods.
A report by the WWF, the global environment campaign, said that last year, at least 40% more tuna was caught in the Mediterranean than is allowed by fishing quotas.
The report blamed French fishermen, as well as those from Turkey, Libya and other countries, for the over-fishing.
A French research institute has also said the fish stocks are being exploited in an unsustainable way, and says the growing fashion for sushi is to blame.