Yes, there was such a group. Actually I had read on Twitter earlier Friday warning people that there would be a group planning to "celebrate" the restart of Ooi Nuclear Power Plant right at the Prime Minister's Official Residence in Tokyo just as the protesters would be gathering.
Asahi Shinbun has a photo of them. (I stand corrected; Asahi did report the protest of June 22 in Tokyo, and they even mentioned 40,000 people. Thank you anon reader for the link.)
From Asahi Shinbun (6/22/2012):
The sign says "Nuclear Power, Our Ally. YES! PRO-NUKE", with the picture of "Astro Boy" - Tetsuwan (strong arm) Atomu (Atom).
This photo contrasts well the "Astroboy" by the small pro-nuke group and a Paul Klee-ish drawing of a girl holding "No Nukes" sign.
It's very interesting that this pro-nuke group put "Astro Boy" on the sign. The author of this extremely popoular manga/TV animation, Osamu Tezuka, didn't like his own creation at all, even though this was what propelled him into the manga stardom in 1952.
Astro Boy has a sister named Uran (Uranium), and a brother named Cobalt.
It's just my hunch and no proof, but I have always suspected that Mr. Tezuka was compelled to create a character like Astro Boy at that particular time, early 1950s. Atomic power is strong, atomic power is good, atomic power is just. That was the message of the Astro Boy manga and animation.
Soon after, an all-out campaign to bring nuclear power to Japan was started by none other than the owner of Yomiuri Shinbun Matsutaro Shoriki, who went on to become the first Secretary General of the Science and Technology Agency (now incorporated into the Ministry of Education and Science).
As nuclear fallout from the atmospheric nuclear bomb testing was circling the globe, Tetsuwan Atomu animation was on the Japanese TV, from 1963 to 1966.
As to the group who did the counter-protest as pro-nuclear, pro-restart, they are rumored to be one of the ultra-right nationalist groups who supposedly hate Korean residents in Japan. (There are much more to this group but that is OT here.)