I saw a Muay Thai tournament back in Moscow in 2000. It was at an arena called (by memory - I could be wrong) Soviet Wings in the northwest part of the city.
There were 16 fighters, but were matched up as follows: 1st seed vs. 16th, 2nd vs. 15th, and so on. This meant that the first few matches were horribly mismatched. Very bloody stuff. One guy got kneed in the face and was obviously unconscious, but took another eight knees before the match was stopped.
I've seen lots of other tournaments there - all kinds of hand-to-hand, no rules stuff - but nothing like that.
I don't know how popular it is now. When I went, it was free to get in. The arena was about half to 3/4 full, mostly of dark-haired black leather jacket Caucasus types.
What's more entertaining is in St. Petersburg at Arena Club. The fights there are far less serious (blood is rare), but really funny. There's a guy there, Sasha, I believe - a big blonde buzz cut guy - who plays to the crowd like crazy. He fights every weekend. If it's boxing, he boxes. If it's Muay Thai, he does that. Sometimes he gets confused - last time I saw him box, he nearly kneed his opponent until he realized he was fighting the wrong style.

I'm pretty sure he was just doing that to make the crowd laugh... hopefully...
Dave