The Metropolitan Police Department put the number of protesters at closer to 20,000 to 30,000, Kyodo News reported.
The rally began in Shiba Park, which was attended by Nobel laureate Kenzaburo Oe, RIA Novosti reported. The protesters then marched on the Diet building.
Participants have gathered more than 8 million signatures against the government's plan to restart nuclear power plants after the 2011 disaster at the Fukushima nuclear power plant due to a massive 9.0-magnitude earthquake, RIA Novosti reported.
A series of explosions at the plant resulted in the worst nuclear disaster since the one in Chernobyl in 1986, RIA Novosti said.
All nuclear power generation facilities in the country were shut down after the disaster.