Two men have been arrested at the Pokemon World Championships venue in Boston, US, following a tip-off by the event's security staff.
Police found several weapons in their car including a shotgun, a rifle, "several hundred rounds" of ammunition and a hunting knife.
The men, who were due to compete in the finals, were named as 18-year-old Kevin Norton and 27-year-old James Stumbo.
They told police only that they had driven to the event from Iowa.
They have now been arrested for unlawful possession of a firearm and ammunition, as well as other firearms offences, Boston Police said.
Superintendent Paul Fitzgerald, commander of the Boston Police District's Bureau of Intelligence and Analysis, praised the private security firm for working with the authorities.
And he went on to describe the arrested pair and their weaponry as a "very real threat".
"The BPD detectives and collaborating agencies did a great job in the stop and prevention of a potential tragedy," he said.
James Stumbo had posted a picture of the pair's car, with two guns on the boot, on Facebook at the start of the week.
The championships, which took place at the weekend, were by invitation only.
Top video and trading card players from more than 30 different countries take part in the annual tournament, competing for top prizes of scholarship money, trophies and merchandise.
Pokemon events attract over 400,000 entrants each year, according to the company behind the brand.
The franchise, which includes card trading and video games made exclusively for Nintendo consoles, has an enormous global following.