Pokemon Go, a mobile game that has become a global phenomenon, has been released in the UK.
It was already available in the US, Australia and Germany but some UK gamers found ways around the country restriction to get early access.
The app lets players roam a map using their phone's GPS location data and catch Pokemon to train and battle.
The game has added millions to the value of Nintendo, which part-owns the franchise.
Demand for the app is so high that new account registrations are periodically unavailable. However, players may still be able to register using a Google account.
"The enthusiastic response to the launch of Pokemon Go has been tremendous and inspiring. We're aware of the server issues that our players have been experiencing and are working around the clock to resolve them as soon as possible," the companies behind the game said in a statement.
The game uses a smartphone's GPS location and real-world maps to track players as they move around. Players can visit Pokestops - typically landmarks or buildings - and collect free items in the game.
The main aim is to catch Pokemon, which pop up along the way. The game provides an augmented reality experience, using the smartphone's camera to provide a live view of the world, with Pokemon superimposed.
Any creatures caught can be used in battle against other players at Pokemon gyms - also real-world landmarks.
However, some people have raised concerns about the app's safety. The chief executive of children's charity NSPCC urged the app's makers to adapt the game before its UK release, warning that adults could use it to prey on children.
An NSPCC spokesman said: "It's deeply troubling that the app's owners have ignored many warning signals and well documented child safety concerns. It would have been better if they had taken time to reflect on these and put their young users first."
The makers of the game - Niantic and The Pokemon Company - said in a statement: "We take player safety seriously and want everybody to have a fantastic time exploring while safely playing Pokemon Go.
"We encourage all people to be aware of their surroundings and to play with friends or family, especially when you're exploring unfamiliar places. Please remember to be safe and alert at all times, don't drive and play, abide by local laws, and respect the locations you visit and people you meet during your exploration."
Top tips for new Pokemon trainers
Choose your starter wisely
At the beginning of the game, you'll be offered a choice of Bulbasaur, Charmander or Squirtle - the three starter Pokemon offered in the first Pokemon games. However, if you ignore the choice and walk away from the Pokemon, you'll get the chance to catch Pikachu instead.
Keep on catching
You'll find the same Pokemon pop up time and time again, especially common creatures such as Pidgey and Rattata. Although you may be determined to "catch 'em all", catching every Pokemon you encounter is a quick way to earn experience and level up, which will give you access to more features in the game. Plus catch enough Pidgey and you'll earn a birdkeeper medal!
Follow the tracks
A Pokemon tracker is available at the bottom right hand side of the screen. Tap it to bring up a list of nearby Pokemon. A greater number of footprints indicates that a Pokemon is further away. Keep the tracker open as you walk around to see the Pokemon change order, with the closest creatures at the top of the list. This should help you find the ones you want.
Pokemon goes worldwide
- An American woman found a dead body while she was looking for a Pokemon in a river near her home. Police said the man had died within the last 24 hours and no foul play was suspected.
- Four people were arrested after they used the game to lure players to remote places and then rob them at gunpoint. In response, the makers of Pokemon Go have said people should "play with friends when going to new or unfamiliar places" and "remember to be safe and alert at all times".
- The anti-gay Westboro Baptist Church in the US is the location of a "gym" in the game, and players planted a pink "Clefairy" Pokemon called Love is Love there. The church has responded with a series of social media posts calling the Pokemon a sodomite.
- There have also been plenty of reports of people falling over and grazing or cutting themselves because they're not paying attention to what's in front of them while they play.