While I was in Tokyo, I checked out Ikebukuro Cosplay Event held at a local park. I was there for less than 2 hours while the event ran for 2 days.
The event was hosted in part by NicoNico and they have a stage with cameras to provide streaming. It is open to the public though in many cases, cosplay events would have a fee for cosplayers and photographers while this one there was no fee - except for the cosplayer for the changing room.
The cosplayers changed at a nearby mall where they pay a changing room fee. I believe it is 500 yen (around $4.50). Information about the event was provided on the website.
The cosplayers could only be in certain area of the streets and park. Usually cosplayers are not allow to enter stores in costume though for this event, there is a map on their website listing specific stores they can enter.
The World Cosplay Summit was there with 22 cosplayers representing 11 countries.
The day's event ended with a short 2 block parade with the WCS representatives. After the parade, cosplayers lined up on the red carpet to pose. I asked some cosplayers later about this. Some cons have red carpets, others do not. You do not have to be on the red carpet as I've seen other cosplayers being photographed on the streets.
Major cosplay social media sites such as CosplayMode, Cure and Cosplay Archive were represented as well as NicoNico who streamed portions of the event.
The best analogy to here in the States would be if a local con set-up a two day event in the park where the public could enter without fee and they set-up a stage, had cosplay skits and walk-ons and Crunchyroll stream the event and the population of San Diego Comic Con happen to attend.
The numbers listed for the event was 250,000 by one news report (link at bottom) which I'm not sure how they came about that number. I paid no ticket nor did I sign into the event. I just walked into a park. 10,000 cosplayers were to have attended. That number can be confirmed because cosplayers had to change in the changing area and pay a fee so there'll be a record of that.
In all, it was an interesting experience and I'm eager to return to Japan to explore more of how cosplay works there.
News articles relating to the event