The free application based on a Nintendo title that debuted 20 years ago has been adapted to the mobile Internet age by Niantic Labs, a company spun out of Google last year after breaking ground with Ingress, a game that merged mapping capabilities with play.
According to the research firm SimilarWeb, the game was downloaded in more than five percent of Android phones in the first two days of release and had outpaced the dating app Tinder.
The game uses GPS and mapping capabilities in mobile phones to let players roam the real world to find "PokeStops" stocked with supplies and hunt cartoon character monsters to capture and train for battles.
PokeStops can also dispense monster eggs, which players incubate by racking up walking distances.
Players can also visit "gyms," where captured cartoon creatures can be conditioned as combatants to seize such training facilities.
"It's cool to actually play as a Pokemon trainer in real life," said Lucas Garcia, a 17-year-old California boy who has been a fan for more than a decade.
"It is nice to have a video game that makes you actually walk around instead of sitting in front of a TV screen holding a controller."
(Also see: Pokemon Go Tips and Tricks)
People who played Ingress will recognize PokeStops and gyms, many being monuments, signs, businesses or other real world spots that served as "portals" that could be captured in the earlier Niantic game.
"We have helped users all around the world have fun, socialize, and get more fit as they play and explore," Niantic chief executive John Hanke said in a blog post when Pokemon Go was released last week in the US, Australia and New Zealand.
A tidal wave of interest in the game has bogged down servers hosting the software, frustrating some players and delaying plans to launch Pokemon Go in more countries.
"I downloaded it because everyone that I know has liked Pokemon since before fifth grade," said California teenager Owen Fairchild, who is now in college.
"Now that I have it, I get it. I never really walked around, but a couple of days ago, I walked across the island... and I hatched two five-kilometer (three-mile) eggs."
A young woman playing Pokemon Go came upon a dead body in a Wyoming river while hunting a water monster in the game, according to US news reports.
Pokemon Go comes with warnings to players to remain aware of their surroundings.
Fairchild told of being so engrossed in the game that he has walked into things on sidewalks.
Some US authorities are advising players to avoid breaking the law by trespassing in places that aren't open to the public in the search for cartoon creatures, and saying that some players have been targeted by criminals.
"If you use this app (or other similar apps) or have children that do, we ask you to please use caution," the O'Fallon, Missouri Police Department said on its Facebook page.
The department also warned that robbers were preying on players drawn to rich troves of Pokemon monsters in parking lots or other places where victims might be vulnerable.
Pokemon monsters can be seen through smartphone cameras, with characters appearing in whatever real settings are in view. They are caught by hitting them with virtual balls tossed by swiping across touch screens.
"I'll walk up to a PokeStop and see all of these people standing around flicking their fingers across their phones," Garcia said.
"It's comforting to know there are a lot of us nerdy types out there."
Pokemon Go is not just heavily downloaded, it is being kept by players and, in the majority of cases, played daily on a scale that already rivals the use of Twitter, according to industry trackers. People are also spending money to buy virtual items.
"I have always liked the idea of going around the real world to collect and catch Pokemon; it has been a dream of mine," Garcia said.
Nintendo, Google and Pokemon Company all invested in Niantic after it spun off from the California-based Internet company.
Nintendo has a stake in the Pokemon Company joint venture that holds the Pokemon copyright.