Thousands of people formed "a human chain" around Japan's parliament complex to demand the government abandon nuclear power after last year's Fukushima crisis.
Sunday's protest in Tokyo was the latest in a series of peaceful demonstrations, including weekly Friday evening protests outside the residence of Prime Minister Yoshihiko Noda.
"We won't allow any more reactors to restart. We want to slam this demand to the government"
- Misao Redwolf, protest organiser
Demonstrators, many wearing gas masks and beating on big, yellow oil-drum drums, filled the streets heading to the prime minister's office and the parliament building, chanting "No to restart! No to nuclear power!" as they held up banners with anti-nuclear slogans.
"After the Fukushima disaster, I thought that the government and vested interests were telling us lies about nuclear power being safe," said Miho Igarashi, 46, an architect from Ibaraki prefecture south of Fukushima.
"We have to raise our voices against" the danger of atomic power, she told the AFP news agency.
Observers said the level of public discontent has grown to levels unseen in decades and crowds have grown steadily since the 'Stop the Nukes' movement began.
Protesters say they are angry that the government restarted two reactors earlier this month, despite safety worries after the multiple meltdowns at Fukushima Daiichi nuclear plant.
All 50 working reactors in Japan went offline in May for "routine checks".
In June, Noda ordered the restart of reactors No 3 and nearby No 4, saying people's living standards could not be maintained without nuclear energy.
Misao Redwolf, an illustrator and one of the organisers, told protesters gathered in Hibiya Park, "We won't allow any more reactors to restart. We want to slam this demand to the government".
Unlike many conventional protests in Japan organised by labour unions and political groups, a large number of independent citizens, including children, have joined the demonstration, with many never having taken to the streets before.
Organisers used Twitter to spread details of the planned protest and aimed their message at ordinary people by banning participants from carrying banners with the names of unions and political groups and creating special areas for families with children.
The first rally on March 29 drew only about 300 people, but the number of protesters has grown since.
The number reached around 200,000 on June 29, the group claimed, but the Metropolitan Police Department released a drastically lower figure of 17,000.
The latest rally comes less than a week after a damning government-backed report on last year's crisis said Japanese officials and Tokyo Electric Power (TEPCO), operator of the crippled Fukushima plant, ignored the risk of an atomic accident because they believed in the "myth of nuclear safety".
A separate parliamentary report said the worst nuclear accident in a generation was a man-made disaster, marked by a lack of oversight and collusion between TEPCO, the government and regulators.
The giant utility largely cleared itself of blame, saying the size of last year's earthquake and tsunami was beyond all expectations and could not have reasonably been foreseen.
A 9.0-magnitude earthquake and tsunami on March 11, 2011, crippled cooling equipment at the Fukushima Daiichi plant, triggering meltdowns that spewed radioactivity and forced tens of thousands of residents to flee.