How would I begin to organise a city-wide game of Assassin?

my first idea would be to create a group on facebook , then invite your friends and other local people who may be interested and tell them to invite friends.. I would go for water guns rather than nerf guns as they would be cheaper to get and cheaper to refill ( plus people can always bring there own). I would recommend like a coloured armband for the players . you may also have to get permission from the local government first. then its up to you and your inner circle to divide up the teams.