The protesters gathered in a park in the shadow of corporate headquarter skyscrapers, a short walk from Ginza, Tokyo's most upscale shopping district.
Hundreds strong, they included workers already laid off as the global downturn battered Japan's economy, and those who feared they might be next.
The demonstrators set off on a march towards Japan's Diet building - or parliament - carrying red flags.
"I support the Communist Party because it's the one that thinks about workers first," said one man.
"We're demonstrating to get better rights for the temporary workers," said another.
"The Communist Party is the only party that gets really serious about problems like this."
Communist ideology has been spread in Japan in unusual ways.
There was a book, Kanikosen - The Crab Factory Ship, which raced back up the bestsellers' lists.
A classic tale of proletarian fishermen uniting to rise up against their bosses, it had been almost forgotten since it was written in 1929.
Publishers have also produced a manga, or comic, version of Das Kapital, Karl Marx's treatise on how capitalism would collapse under the weight of its own contradictions.
One new Communist Party member we met in a restaurant found out about Marxism on the internet.
"I got interested in Karl Marx a few years ago," she said.
"In capitalism now we are controlled by the capitalists, or capital. But I think in communism society we can think about whole of the society and decide our economic activities in democratic way."
The woman, 34, did not want to be identified for fear her employers, whom she claimed disapproved of the Communists, would find out.
But she had told her family.
"My parents were very surprised that I joined the party," she said. "They are not supporters of the Communist Party. They don't understand correctly, I think."
The woman said she was a member of a "lost generation" - people who came into the employment market during Japan's long stagnation in the 1990s and could not find proper jobs.
...see
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