TOKYO - Thousands of Japanese marched to celebrate the last of this nation's 50 nuclear reactors switching off Saturday, shaking banners shaped as giant fish that have become a potent anti-nuclear symbol.

Japan will be without electricity from nuclear power for the first time in four decades when one of three reactors at Tomari nuclear plant in the northern island of Hokkaido goes offline for routine maintenance checks.

After last year's March 11 quake and tsunami set off meltdowns at Fukushima Dai-ichi plant, no reactor stopped for checkups has restarted amid growing public worries about the safety of nuclear technology.

"Today is a historical day," shouted Masashi Ishikawa to a crowd gathered at a Tokyo park, some holding traditional "Koinobori" carp-shaped banners for Children's Day that have grown into a symbol of the anti-nuclear movement.

"There are so many nuclear plants, but not a single one will be up and running today, and that's because of our efforts," Ishikawa said.

The activists said that it was fitting that the day Japan will stop using nuclear power coincided with the nation's annual Children's Day, because of their concerns about protecting children from radiation, which Fukushima Dai-ichi is still spewing into the air and water.

The government has been eager to restart nuclear reactors, warning about blackouts and rising emissions as Japan is forced to turn to oil and gas for energy.

Japan now requires reactors to pass new tests to withstand quakes and tsunami and needs local residents' approval to restart them.

The response from people living near the nuclear plants has been mixed, with some wanting them back in operation because of jobs, subsidies and other benefits to the local economy.

Major protests, like the one Saturday, have been generally limited to urban areas like Tokyo, which had gotten electricity from faraway nuclear plants, including Fukushima Dai-ichi.

Before the nuclear crisis, Japan relied on nuclear power for a third of its electricity needs.

The crowd at the anti-nuclear rally, estimated at 5,500 by organizers, shrugged off government warnings about a power shortage. If anything, they said, with all the reactors going offline one by one, it was clear the nation didn't really need nuclear power.

Whether Japan will suffer a sharp power crunch is still unclear.

Electricity shortage is expected only at peak periods, such as the middle of the day in hot weather, and critics of nuclear power say the proponents are exaggerating the consequences to win public approval to restart reactors.

Hokkaido Electric Power Co. spokesman Kohei Ofusa said Saturday's shutdown was going ahead as planned. Power generation is gradually being turned down from 5 p.m. (0800 GMT) with all operation ending at 11 p.m. (1400 GMT), he said.

Yoko Kataoka, a retired baker and grandmother, who was dancing to the music at the rally waving a small paper Koinobori, said she was happy the reactors were going off.

"Let's leave an earth where our children and grandchildren can all play without worries," she said, wearing a shirt that had, "No thank you, nukes," handwritten in the back.

Loading Slideshow

• People gather at an anti-nuclear demonstration on the Children's Day national holiday, calling for a safer future for younger generations at a park in Tokyo on May 5, 2012. The last working reactor in Japan is to be switched off May 5, 2012, leaving the country without nuclear power just over a year after the world's worst atomic accident in a quarter of a century. AFP PHOTO / KAZUHIRO NOGI

• People gather at an anti-nuclear demonstration on the Children's Day national holiday, calling for a safer future for younger generations at a park in Tokyo on May 5, 2012. The last working reactor in Japan is to be switched off May 5, 2012, leaving the country without nuclear power just over a year after the world's worst atomic accident in a quarter of a century. AFP PHOTO / KAZUHIRO NOGI

• People gather at an anti-nuclear demonstration on the Children's Day national holiday, calling for a safer future for younger generations at a park in Tokyo on May 5, 2012. The last working reactor in Japan is to be switched off May 5, 2012, leaving the country without nuclear power just over a year after the world's worst atomic accident in a quarter of a century. AFP PHOTO / KAZUHIRO NOGI

• People gather at an anti-nuclear demonstration on the Children's Day national holiday, calling for a safer future for younger generations at a park in Tokyo on May 5, 2012. The last working reactor in Japan is to be switched off May 5, 2012, leaving the country without nuclear power just over a year after the world's worst atomic accident in a quarter of a century. AFP PHOTO / KAZUHIRO NOGI

• Protesters stage an anti-nuclear demonstration on the Children's Day national holiday, calling for a safer future for younger generations at a park in Tokyo on May 5, 2012. The last working reactor in Japan is to be switched off May 5, 2012, leaving the country without nuclear power just over a year after the world's worst atomic accident in a quarter of a century. AFP PHOTO / KAZUHIRO NOGI