Sea Shepherd says one of its ships has been set on fire and pelted with rocks during a violent confrontation in a marine refuge in Mexican waters.
The conservation group said the M/V Farley Mowat was in the Gulf of California on a mission to protect the vaquita porpoise, one of the world's most endangered marine mammals.
Sea Shepherd said the ship was approached by about 50 poachers on 20 high-speed boats armed with large stones and lead weights.
"Windows were shattered by stones that were hurled at the ship, and the hull was set on fire by a Molotov cocktail thrown by the poachers," a Sea Shepherd spokesperson said.
Crew members were forced to defend the ship with high-pressure fire hoses while members of the Mexican Navy and Federal Police aboard the ship reportedly shot into the air to deter the attackers.
There were no reports of injuries sustained by the crew or security personnel.
No arrests were made, a Sea Shepherd spokesperson said.
The conservation group posted video of the confrontation on Facebook and YouTube.
The clip included vision which appeared to have been posted to social media by the attackers, showing a large boat on fire.
"We have never thrown Molotov cocktails or rocks at another ship, nor have we lit another vessel on fire," a Sea Shepherd spokesperson said on Facebook.
"These are not our tactics."
The Farley Mowat was on a mission to protect the Vaquita Marine Refuge, which has suffered from overfishing in recent years, with totoaba fish attracting poachers and criminal gangs to the area.
Totoaba swim bladders sometimes fetch thousands of dollars per kilo for use in traditional Chinese medicine.
A 2018 report from the Elephant Action League found dried totoaba fish bladders were being smuggled out of Mexico via indirect flights to China, often carried in checked luggage.
The report said the bladders were priced higher than gold and the trade was considered lower-risk than drug ventures, with at least three major cartels illegally trafficking the fish.
Totoaba cartels target the highly-prized fish with large gillnets, which have been banned in the area since May 2015.
Along with killing the endangered fish, the nets also trap and kill other wildlife, including vaquita porpoises.
According to a report from Environmental Investigation Agency, there were fewer than 30 vaquita porpoises in the area in July last year.
