A lot of them have been running for quite a while!
According to TFWiki, there are two official ones: BotCon in North America (since 1994) and Cybertron Con, its counterpart in Asia since 2010 (China and Singapore, so far).
Then there are plenty of unofficial ones: http://tfwiki.net/wiki/Unofficial_conventions
The biggest in Europe are Auto Assembly UK, Auto Assembly Europe and C.O.N.S., from the sound of it. There is also a Dutch/Belgian one, B.O.T.S., which is attended by Hasbro representatives, or so I'm told.