Then the army would declare war and kill them all.
Seriously, it won't be like in the cartoon. I mean, how would the police feel if a gyarados rampaged through a city? And remember, pokemon aren't invincible.
Come on now, imagine if Pokemon was the kind of animals we had in this world, instead of the real animals. Imagine living in a world of the same kind as depicted in the anime, with tournaments, travelers, breeders and so on... Actually it wouldn't be that different. Humans are adaptable critters.
And there is solutions to rampaging Pokemons... You know, they have trouble with them in that world too...
Why are so many of you saying it would end horribly?