LAS VEGAS -- Graffiti vandals have defaced the world famous “Welcome to Fabulous Las Vegas” sign on the southern end of the Las Vegas Strip.

The graffiti, which appeared to be written with a marker rather than spray paint, was spotted Monday. Several letters were written on the sign, but no profanities.

Russell Davis of Clark County Public Works said the graffiti will be cleaned up Tuesday morning. He said this was the first time he could remember that graffiti had been tagged on the famous sign.

Davis said sign maintenance workers discovered the graffiti, which they believe happened sometime over the weekend. Every day, hundreds stop to have their photographs taken at the sign, making it highly unusual that a person would be able to deface it without being seen.

Although the sign falls outside city lines, Las Vegas Mayor Goodman issued a statement to FOX5 saying, "When they [defaced] my tortoise, I said, 'off with their thumb!' This deserves, 'off with their head!'"

Goodman was referring to a time in 2005 when he suggested police cut the thumbs off vandals who defaced a tortoise statue near downtown Las Vegas.

Built in 1959, the Las Vegas sign has become a symbol of the city, appearing in ad campaigns, films and even board games. In May, the sign was officially welcomed onto the National Register of Historic Places.