SAO PAULO—Sao Paulo is creating a registry of street art to be preserved, Bloomberg reports. The move comes after artists complained about city clean-up efforts that resulted in the painting over of part of a mural by twin brothers Otavio and Gustavo Pandolfo, known as Os Gemeos.

Os Gemeos were invited by the Tate Modern to participate in this summer's street art show, in which artists painted murals on the museum's exterior. Only one month later, part of the duo's 2,230-foot mural on retaining walls on the 23 de Maio expressway in Sao Paulo was covered by gray paint, despite the fact that it had been done with permission from the city.

Officials blame the incident on a misinterpretation of the Clean City law. Enacted in 2006, the law is Mayor Gilberto Kassab's attempt to decrease "visual pollution" in Sao Paulo. It has led to the removal of billboards, the scaling back of signs with corporate logos, and the covering over of much graffiti.

"Outside of Brazil, graffiti art has been much more accepted," Gustavo Pandolfo told Bloomberg. "Galleries and museums invite us to do shows. And in Sao Paulo, where we do this mural for free as a present to the people of the city, it's viewed as trash."

According to Regina Monteiro, director of projects, environment, and urban landscaping in Sao Paulo and the person in charge of coordinating the city clean-up, some inspectors "understood the Clean City law to mean paint over anything that's irregular. Because the law didn't give objective criteria, it was left up to subjective opinion."

Monteiro says they are now developing those criteria, making exceptions for graffiti that was painted with permission from the property owner. The city expects to have the registry ready by November.