Not surprising, given the summer vacation and coming "obon".
More and more people have started to openly say how immature some of the organizers of the Friday protests are in attacking anyone who doesn't agree with them with vicious words. Some also say women from Fukushima Prefecture had been protesting in the Kantei (PM's Official Residence) area long before they showed up.
Attribution errors on the part of the organizers (i.e. they are the ones who have made it happen) may cost dearly.
Photographs that I found on Twitter seem to show there was a plenty of space and a plenty of policemen but not enough protesters to fill the space and keep the police busy. There is no mention of how many people participated, either from the semi-official police source or from the organizers. No one is even guessing (or not that I could find).
From Akahata (Japan Communist Party's paper):
From "ざまあみやがれい" blog, screen shot of IWJ's Channel 5 at the start of the protest:
Photos from Labor Net Japan at least show people carried all sorts of messages, and it was no longer the "single issue" protest. Some are calling for evacuating children from Fukushima, others are calling for the ousting of Noda, and yet others are against the new Nuclear Regulatory Commission or against sales tax hike:
The woman in the last photo with tattoos on both arms is one of the organizers.