The message appeared shortly after 7 p.m. on the bus' internal driver display, which usually provides information about traffic conditions and accidents.
A driver who spoke to the newspaper said many colleagues were offended by the message. "I think it's degrading. I've never seen anything like it before," he said.
Bus operator Arriva confirmed that the message was sent but they are not sure how it could happen or who was behind it. The matter will be investigated internally.
"Well we have our suspicions, but there is nothing I want to comment before the investigation is complete," said Tomas Hedenius, Arriva's spokesman.