Thailand's leading university has apologised for displaying a billboard that showed Adolf Hitler alongside Superman and other superheroes, saying it was painted by ignorant students who did not realise the image would offend anyone.

The huge billboard was placed outside the art faculty of Chulalongkorn university as part of a tribute to this year's graduates.

It said "Congratulations" in bold white letters and showed Hitler with his arm raised in a Nazi salute next to Batman, Captain America, the Incredible Hulk and Iron Man.

"[We] would like to formally express our sincere apology for our students' superhero mural," the art school dean Suppakorn Disatapundhu said in a statement on Monday. "I can assure you we are taking this matter very seriously."

The billboard was up for two days before being removed on Saturday in response to criticism. Online photographs showed graduating students in their robes, mimicking Hitler's raised-arm salute.

Suppakorn said new art students had painted the banner as part of a traditional send-off from incoming students to the graduating class, and it was one of dozens of banners and billboards across the campus during the university's commencement period.

The artistic vision behind the picture was to show that good and bad people co-exist in the world, Suppakorn said after summoning the students for an explanation.

"They told me the concept was to paint a picture of superheroes who protect the world," the dean said in a telephone interview.

"Hitler was supposed to serve as a conceptual paradox to the superheroes," he said, noting that the superheroes were painted in vivid colours while Hitler's image was in grey. "This kind of thoughtless display will not happen again."

The Simon Wiesenthal Centre, an international Jewish human rights group, had criticised the banner before its removal.

"Hitler as a superhero? Is he an appropriate role model for Thailand's younger generation – a genocidal hatemonger who mass-murdered Jews and Gypsies and who considered people of colour as racially inferior?" said Rabbi Abraham Cooper, associate dean at the centre, in a statement on Friday.

"The Simon Wiesenthal Centre is outraged and disgusted by this public display at Thailand's leading school of higher education."

The study of history in the Thai school system revolves primarily around the history of Thailand and its long line of kings. World history is glossed over, with little or no mention of the Holocaust.