That's the largest crowd so far, protesting in front of the Prime Minister's Official Residence.
It looks to be a combination of long-time anti-nuclear activist organizations and ordinary citizens. Unlike the protests that take place there regularly these days, the one last night (June 8, 2012) was covered by the MSMs including Mainichi, Asahi, NHK, Kyodo News and AP, and not so MSM Tokyo Shinbun, according to one tweet. Another tweet says there were also Reuters and "Red Flag" - official party newspaper of the Japan Communist Party who has been vigorously covering the nuclear accident, resultant contamination, and Japan's nuclear policies since the accident.
As Mainichi Shinbun reports (6/9/2012; link added):
Ooi Nuke Plant restart: 4000 people protest in front of the Prime Minister's Official Residence, as Prime Minister gives a press conference
On June 8 in front of the Prime Minister's Official Residence in Nagata-cho in Tokyo, about 4,000 citizens protested as Prime Minister Yoshihiko Noda gave a press conference regarding the restart of Reactors 3 and 4 at Ooi Nuclear Power Plant (Ooi-cho, Fukui Prefecture). Yuki Yoshikawa (age 43), a housewife in Asahi-ku in Yokohama City who was originally from Kansai, came with her 3 year-old son. "Ooi Nuclear Power Plant is close to the Lake Biwa, important water source for Kansai Region. Why wouldn't [the prime minister] listen when this many people are against the restart?" she said.
Terumi Tanaka, Secretary General of the Japan Confederation of A- and H-Bomb Sufferers Organizations, said, "Restart of Ooi means the first step to continued reliance on nuclear power. If an accident happens, citizens will be made responsible by having to pay [increased] taxes and utility fees."
The Council of Mayors Against Nuclear Power Plants, an organization made up of mayors of 68 municipalities in Japan [set up on April 28, 2012] pointed out, "Total overhaul of the organizations that have lost credibility in assessing the safety should be the first priority, including the Nuclear Safety Commission. It is not right to have the same, pre-Fukushima organizations evaluate safety."
But the governor of Fukui Prefecture, Issei Nishikawa, is happy with Prime Minister Noda's speech, and he says he will take necessary steps within his prefectural government next week for the quick restart of Ooi Nuclear Power Plant, NHK tweets.
The same NHK (Kabun) also tweets that 57% of municipalities within the 30-kilometer radius from Ooi Nuke Plant responded to NHK that they do not have effective measures in the event of a nuclear accident at the plant.
The Noda administration seems to be taking steps to discourage a gathering in front of the PM Residence, however. Someone spotted this sign that says "Sidewalk repair work to be carried out, from late June to December 21, 2012". (Disclaimer: I have no way of knowing whether this sign was definitely in front of the PM's Official Residence.)