The concert was sponsored by a beer maker and held on a field often used for such events.
While the Guatemalan rock band Bohemia Suburbana closed the show, some concertgoers were crushed as some tried to leave as others were entering the same place.
Nancy Quemé, who was at the concert, said there had been thousands of people there.
"Because of the rain there was a lot of mud," she said.
"I think because of this the people couldn't move and they fell."
The line-up of bands had started playing on Wednesday afternoon.
She said that even in the early hours of Thursday there were still families with children there.
"They closed off the whole area and only left two access (points)," Quemé said.
"The entrances seemed really small to me. I stayed pretty far back and decided to leave minutes before the end of the concert."
Video circulating on social media platforms shows dozens of people smashing into others.
Shouts can be heard calling for people to stop pushing and to move to one side so those who fell could be rescued.
Amilcar Rivas, Quetzaltenango city manager, said that event organisers did not have a grip on security and crowd control.
He said the event did have a permit.
Quetzaltenango, which holds Guatemala's second largest independence celebration, is about 200km west of Guatemala City.
Guatemala is celebrating 201 years of independence from Spain on Thursday.
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