The largest event was in 2005, Eger - when we had 77 participants (in the beginning there was only 14).
The first championship was in 2000. Some photographs of this championship can be found at Gyorgy Istvan's Picasa account. It was organized every year always in Budapest - with few exceptions, when 24HPC was held the right after the World Puzzle Championship (WPC) in Eger.
Originally, there was an international Crossword Puzzle Compiler championship in the 1980's between the Eastern European countries, named Puzzle Marathon - where each country compiled crosswords in own language (on the given sheets there was some given well-known person and town-names, so they have no opportunity to prepare puzzles in advance). And that lasts for 24 hours. In 1999, at the WPC in Istanbul. Mr. Ede Markos (member of the Hungarian Team in 1999-2000) came up with the idea to do something similar to the Puzzle Marathon, but with logical puzzles. Then after a short discussion it was agreed to organize this event. In the first two years there was only Hungarian participants, later this event became an international event. And now beyond the World Puzzle Championship and World Sudoku Championship it became the largest on-spot yearly international championship.
In each year there at 10-15 organizers who correcting the puzzle sheets and giving the result within a certain time. Some of them working for 24 hours - just like the puzzlers.
Thanks to István György providing above mentioned interesting information regarding this championship.
In year 2015, The Puzzle Marathon will be held again in Budapest, 17-19 April, 2015. In this year Pál Madarassy will be again in charge of puzzle director position. More information about this championship can be found at official Facebook page for this championship.
From India myself along with Prasanna, Rakesh, Ashish and Swaroop are providing two set of puzzles for this championship. Looking forward to participants feedback for this championship.