TOKYO — In Japan’s largest antinuclear rally since the disaster at Fukushima, tens of thousands of protesters gathered at a park in central Tokyo on Monday to urge the government to halt its restarting of the nation’s reactors.
Organizers said 170,000 people filled a Tokyo square to sing songs, beat drums and cheer on a series of high-profile speakers who called for more Japanese to make their voices heard. The police put the number at 75,000, still making it the biggest gathering of antinuclear protesters since the Fukushima accident last year.
“To stay silent in the wake of Fukushima is inhuman,” the Oscar-winning musician Ryuichi Sakamoto told the crowd, which braved soaring temperatures to gather at Yoyogi Park.
Polls suggest that public opinion is still divided over the future of nuclear power in Japan. But a unilateral decision last month by Prime Minister Yoshihiko Noda to start putting the country’s reactors back into use has angered many Japanese and galvanized the antinuclear camp.
Antinuclear protests have gained momentum especially here in the capital, where tens of thousands of protesters now gather every week to shout slogans in front of Mr. Noda’s official residence.
After the rally, protesters marched through some of the city’s busiest shopping districts, prompting curious looks from passers-by but largely maintaining a strict discipline that has come to characterize the antinuclear rallies in recent weeks.
“It doesn’t matter, for now, if people hear us or not,” said Ayuko Higashi, an illustrator from Kamakura, southwest of Tokyo, who said this was her third antinuclear rally. “It’s just a big step forward to start raising our voices.”
Rally organizers have gone to great lengths to project a friendly image in a generally conformist country where protesters of any kind are seen by many as fringe agitators at best and terrorists at worst. This perception is left over from mass protests in the 1960s and ’70s against a security treaty with the United States, during which rioters armed with pipes and makeshift gasoline bombs clashed with the police.
At the weekly protests in front of the prime minister’s office, organizers cordon off family-only zones to urge parents with children to participate. They also ask protesters to cooperate with the local police and to go home at 8 p.m. on the dot.
Organizers have also started issuing pamphlets with advice on what to bring (drinks and hand wipes on sticky days), advice for shy or first-time participants (no need to say anything) and guidance on what to do if fellow demonstrators start getting out of hand (politely ask them to calm down).
An unlikely leader of the antinuclear movement is a fuzzy fictional character called Monju-kun, who has amassed a following on social networking sites like Twitter for his child-friendly jabs against the government’s energy policy.
Monju-kun made a brief appearance at the protest Monday with a colorful stuffed costume. “The government is restarting our nuclear reactors, and that makes me sad,” he said to squeals from fans — many of them families with small children — waving Monju-kun posters.
“If we’re not careful, we could have another nuclear accident, like a fire,” said Rion Nakajima, 5, who was clutching a balloon emblazoned with a smiling Monju-kun.
“We decided to participate because we want the Japanese government to realize that any mistake it makes now will have serious repercussions for future generations,” said Rion’s mother, Kazuki.
Nuclear power supplied nearly a third of Japan’s electricity before the Fukushima accident, but almost all reactors are now offline for checks or maintenance. Japanese leaders pledged last year not to restart any nuclear plant without local approval. But they apparently did not foresee the level of resistance that has since developed, and they have begun to push for plant reopenings, citing economic and security reasons. Last month, Mr. Noda approved the restarting of a reactor in western Japan. Others are expected to be put back into use in the coming months.
Recent polls have shown that public opinion remains divided between those who argue that Japan should abandon nuclear power and those who warn of a crippling energy shortage. A majority favor more stringent checks of reactors.
It is unclear whether the antinuclear protests can become a political force. There is still no significant Green Party in Japan, and the two largest parties say the country, for now, needs its reactors.
Mr. Noda, who initially raised the ire of protesters by calling their rallies “loud noise,” said last week that he was fully aware of public opinion both for and against nuclear power.