It was the latest twist in the saga of Tama-chan, a bearded seal who showed up in August in the Tama River near Tokyo, more than a thousand miles from his Arctic habitat.
Japanese have embraced the slippery creature with passion, buying Tama-chan action figures and making him an honorary citizen of the Tokyo suburb of Yokohama.
Amusement turned to anger Tuesday when a citizens' group concerned about Tama-chan's welfare enlisted the Marine Animal Lifeline, based in Westbrook, Maine, to capture him.
The plan was to return the seal, now living in a polluted Yokohama river, to the northern Sea of Okhotsk, but the American group failed to snare the seal after sending four divers into the river.
Instead, about 20 of their supporters got caught in an emotional standoff with dozens of residents who gathered along the shore in protest, said Teijiro Saito, a local government official.
One woman was reduced to tears as the two sides traded recriminations, and police arrived to calm tempers.
"Tama-chan is adapting to the environment of the river. Why meddle?" resident Akio Fujii told Kyodo News agency.
Authorities accused divers of lying about their intentions. They had told river management officials the dive was to conduct environmental research, Saito said.
The group denied doing anything illegal, saying it was only trying to help local activists.
"Our position was not to be an advocate either way," member Greg Jakush told The Associated Press, adding that the seal appeared to be in good health.
"Our primary goal was to observe," he said. "We've completed what we've intended to come out here to do."
Though reports from The Mainichi Daily News, a Japanese English-language daily, indicated some members took a less neutral stance.
"Tama-chan is not a pet, he's a wild animal," one Marine Animal Lifeline member was quoted as saying. "The best thing to do is save him while he's healthy and return him to the sea.
Two group members returned home Tuesday. Jakush was to leave with the fourth member Wednesday.
An official at the Kanagawa prefecture government who spoke on condition of anonymity said the seal's fate was "not a legal matter" but warned the group against "just doing whatever they happen to think is right."
A government-appointed committee of experts has decided to leave Tama-chan free to frolic in Yokohama's murky, temperate rivers unless they suspect he is ill or injured.