Banging on drums and waving balloons and banners, protesters marched from a Tokyo park and lined up along the blocks around the parliament building, chanting, "Saikado hantai," or "No to restarts," and later lit candles.
"All these people have gotten together and are raising their voices," said Shoji Kitano, 64, a retired math teacher, wearing a sign that said: "No to Nukes."
Kitano said he had not seen such massive demonstrations since the 1960s. He stressed that ordinary Japanese usually don't demonstrate, but they were outraged over the restarting of nuclear power.
Similar demonstrations have been held outside the prime minister's residence every Friday evening. The crowds have not dwindled, as people get the word out through Twitter and other online networking. A July 16 holiday rally at a Tokyo park, featuring a rock star and a Nobel laureate, drew nearly 200,000 people.
The crowd appeared to be smaller Sunday. Kyodo News service estimated it at 10,000 people. Participants said they came from across Japan, underlining the widespread appeal of the protests.
Prime Minister Yoshihiko Noda defended his decision to restart the two reactors at Ohi nuclear plant in central Japan as necessary to maintain people's living standards. Other reactors are also expected to go back online, one by one.
Reports from government and legislative investigations have been released on the Fukushima disaster, including a recent one that blamed "the Japanese mind-set," which it said had allowed collusion between the plant's operator and regulators. The reports have done little to allay people's fears.
Adding to protesters' frustrations is the support nuclear power has gotten from regional governments, where the plants are located. They said they planned to vote antinuclear candidates into office to effect change.
How the antinuclear candidate in Yamaguchi Prefecture fares in Sunday's election is critical in possibly signaling a break from the past. The state is home to relatively poor rural and fishing areas.
Such places, far away from the capital of Tokyo, have been typically chosen to house nuclear plants, with residents won over with jobs and subsidies.
There is a plan to build a nuclear plant in Yamaguchi, but doubts are growing over whether that can be carried out.
Tetsunari Iida, the Yamaguchi candidate, is against that plan and nuclear power in general.
"We can change history," Iida wrote in an online message, criticizing "the nuclear village," the term here that refers to the collusion between the industry and the government. "That is my choice."