Residents of the Santa Elena in Spain's southern Jaén province are reeling after news of the damage.
Local mayor Juan Caminero said the painting was now "irreparable" and condemned the act of vandalism as "heartless", Spanish daily La Vanguardia reported on Monday.
News of the attempted theft first emerged on Saturday after visitors to the Los Escolares Cave noticed the damage to the zoomorphic painting of a person resembling a swallow.
After noticing what looked like evidence of someone having tried to chip away at the image with a pick, they spotted fine dust and rock fragments on the floor.
Police are now investigating the incident.
Jaén province is home to 42 sites with UNESCO-listed rock paintings, many of them freely accessible to the public and without not properly protected, according to the Speleology Federation of Andalusia (Fae).
These paintings are part of a significant group of several hundred sites along Spain's Mediterranean seaboard, and are the largest group of rock sites in Europe, according to Unesco.
"A lot of these places are abandoned and need greater supervision," Fae president José Antonio Berrocal told The Local.
"Although there is legislation protecting these sites in theory, there is a lack of political will," he added.
"Obviously you can't have policemen stationed in every cave, but we need a system of continuous monitoring with officers coming around periodically to monitor the situation," the cave expert explained.
"In some cases, closing off those caves may be the only option to protect world heritage paintings," he added.
The Spanish rock sites display "a critical phase of human development...in paintings whose style and subject matter are unique," Unesco says on its website.