Thousands of people have formed "a human chain" around Japan's parliament complex to demand the government abandon nuclear power - the latest in a series of peaceful demonstrations that are on a scale not seen for decades.
Voters also went to the polls in a closely watched regional election for governor in south-western Yamaguchi Prefecture, where an outspoken anti-nuclear candidate is running.
Protesters said they were angry the government restarted two reactors earlier this month, despite safety worries after the multiple meltdowns at Fukushima Dai-ichi nuclear plant in March last year. They were the first to come back into operation since May when the last of Japan's 50 working reactors went off-line for routine checks.
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 maths teacher, wearing a sign that said: "No to Nukes". Mr Kitano said he had not seen such massive demonstrations since the 1960s. He stressed 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 on Sunday. Kyodo News service estimated it at about 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 anti-nuclear candidates into office to effect change.