Twenty people were shut out of Longstone Cemetery in St Ives before being let in at lunchtime on Christmas Day, with some stuck outside for hours.
Cormac, which maintains the cemetery for the town council, said its staff normally open the gates when they clean the toilets each morning.
The firm said unlocking the gates was "missed" because the toilets were shut.
Martina Anscombe attended the cemetery with a bouquet for her parents' grave.
"There were quite a few cars there already, a few distraught people that couldn't get in... forty five minutes later there were a lot more distressed people there, especially elderly people trying to get in because their wives or husbands had passed away," she said.
"About ten minutes after I left I heard that someone had actually come with bolt cutters as they were so upset."
A spokesman for Cormac said: "The unlocking of the cemetery is a free service we provide as we clean the public toilets. As the toilets were closed on Christmas Day then it was unfortunately missed.
"We are really sorry for any distress that this caused and we will be discussing this with St Ives Town Council to ensure that this doesn't happen again."
The cemetery belongs to Cornwall Council and is maintained by St Ives Town Council which has sub-contracted the upkeep and security to Cormac for two years.
"Distraught" mourners were forced to scale the gates of a graveyard to lay flowers after they were left locked.