Argentine media said around 50 supporters invaded the practice and demanded the relegation-threatened team win their last three games.
"They started to argue and threaten us," San Martin's Chilean striker Cristian Canio (pictured, left) said in an interview with the ADN radio station in his homeland.
"They had a go at the coach because they said he played too defensively. Then they continued with us, they said we were robbing the club.
"Then, they got out their guns, showed them to us and when they were leaving, they fired a shot into the air.
"They said that if we didn't win the nine points we still have to play for, we would have to face the consequences."
San Martin, beaten 3-0 at Boca Juniors on Sunday, are 19th in the relegation standings which are decided over three seasons. The bottom two in the 20-team table go straight down.
Argentine teams are followed by organised groups of hardcore fans known as "barras bravas" who are often blamed for violence in and around stadiums.
Groups of fans often demand to speak to the players when they are not satisfied with performances, sometimes making threats.
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