Pokemon Go servers appeared to crash on Saturday, leaving millions unable to play the augmented reality game.
Users across Europe and the US complained they are unable to access the game, or that it is regularly freezing.
Players began to report problems with the game shortly after 2pm.
A hacking group named PoodleCorp has claimed responsibility for taking down the Pokemon Go servers through a denial of service (DDOS) attack.
The problems follow the US launch of the game earlier this month, which caused servers to crash due to overwhelming demand.
The game was launched in the UK on Thursday.
The makers of the game issued a statement on Twitter on Saturday afternoon saying: "We have been working to fix the #PokemonGO server issues. Thank you for your patience."
Frustrated players took to social media to complain about the problems.
By Saturday night many most players were able to access the game.
The incident is one of many international news items relating to Pokemon Go since the launch of the game earlier this month.
A man crashed his car into a tree in New York on Friday, later admitting to police he had been distracted by the game.
On Thursday two men in San Diego fell off a cliff after they climbed over a fence in an attempt to catch the animated characters.
Fish out of water: Magikarp found in unusual places on Pokémon Go
Last week a teenager from Wyoming found a dead body in a river after she climbed over a fence to go in search of Pokémon near her home.
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