The first ever, official Pokémon Go Fest collapsed into chaos on Saturday, after attendees who had queued for hours for the chance to catch Legendary Pokémon Lugia found the game almost unplayable due to overloaded mobile phone networks.

Developer Niantic has apologised to fans, offering a full refund on the $20 ticket, $100 worth of in-game Pokécoins, and giving a Lugia to all registered attendees, but the event still casts a shadow over what was supposed to be a very public celebration of the game’s first anniversary.

Taking place in Chicago’s Grant Park, Pokémon Go Fest featured real-world amenities like photo opportunities with Pokémon and talks from Niantic employees. But the big draw was supposed to be in the virtual world: challenges for the three Pokémon Go teams – Valor, Mystic and Instinct – a medal for visitors and a plethora of rare Pokémon in the area, in addition to the chance to be among the first in the world to capture the first of the new legendary Pokémon to come to the game called Lugia.

But trouble started almost immediately. From 6am on the day – four hours before the event was due to start – players were complaining they were unable to log in, as the concentration of high-bandwidth connections swamped, then collapsed, mobile networks in the area.

By 2pm, Niantic had acknowledged the issue, though not in time to prevent its chief executive being booed as he took to the stage, to chants from the audience of “we can’t play!”

the CEO of niantic getting booed on stage at pokemon go fest brings me nothing but joy pic.twitter.com/6WxTAvv76Q — Ｚ Ｅ Ｆ (@therealzef) July 22, 2017

“This is not the day that we had all envisioned, but we appreciate your patience,” chief marketing officer Mike Quigley told the crowd shortly after.

The company also extended the radius of the area containing the rare Pokémon, to encompass a two mile space outside Grant Park for the 24 hours of the festival, in order to try and ease congestion on the phone networks.

Trainers, we’re aware of server and connectivity issues impacting #PokemonGOFest and are working as quickly as possible to address them. — Pokémon GO (@PokemonGoApp) July 22, 2017

“I spent more time trying to get the game to load than I have playing it,” one attendee told The Verge.

With the festival over, Pokémon Go fans are now free to try and find Lugia worldwide. A second legendary Pokémon, Articuno, has also been added to the game.

Both Pokémon are caught rather differently from traditional ones, with a number of players in the same area needing to join in with a “raid”, and fight en masse to take down the legendary Pokémon. The raids are joinable for up to two hours, and if enough players take part and win the fight, they’ll all have the opportunity to throw Pokéballs at the legendary Pokémon to try and catch it.