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The protest is because the prime minister (Abe Shinzo) is trying to get a few bills through parliament that would allow Japan to participate in collective self-defense. In particular, this would mean that they could have a pact with the US. The US is currently at war with several abstract concepts and some entities that are not nation states, so what does collective self-defense mean in this case? It also seems to be contrary to article 9 of the Japanese constitution. Even if the bills pass, they will be quickly challenged in the Supreme court, where hopefully they'll be trashed. Of course Abe's party has a proposal to fix the constitution too.
For extra fun, she was being interviewed by CCTV, the Chinese state TV station.