Pokemon fans in Japan can at last grab their smartphones and set off outdoors in pursuit of monsters as "Pokemon Go" debuted Friday.

After it launched overseas on July 6, the GPS-based smartphone game has already become a massive hit in more than 30 countries. The unexplained delay in releasing it in Japan left die-hard fans perplexed and itching to go.

"Pokemon Go," a game that is free to download and play, uses the phone's GPS sensor and camera to guide players around streets and parks to find and capture Pokemon monsters. The characters are superimposed on a live camera image of the surrounding scenery, offering an immersion experience that many find addictive.