Between 5,000 and 7,000 people are though to have descended on Madrid’s Puerta del Sol at 19:00 local time (18:00 BST), to catch Pokemon together and fight other players.
The location of the event was moved from the Retiro park, after the Madrid City Council warned that such a large-scale event could damage the environment.
The organisers explained on their website that there is better mobile coverage in the Puerta del Sol, and the plaza has two “gyms” where players can train and fight their Pokemon.
“There will also be bait installed to capture the largest number of Pokemon possible”, the event’s organiser Fever told Spanish newspaper.
However, many gamers have reportedly chosen to ignore the advice of the council, and have headed to the Retiro park anyway.
There have already been several unofficial gatherings of Pokemon Go players in the Retiro park, without the permission of the city council.
Rita Maestre, a spokeswoman for the City Council, said that players who do head to Retiro should take “special care”, as the park is a protected cultural attractions space.
Pokemon GO is a free augmented reality game that lets you capture virtual Pokémon hidden in real-world locations.
It has proved wildly popular with gamers all over the world – and there are even plans to make it into a Hollywood film.