Polish police detained a 58-year-old man after he threw vials of black paint at the Black Madonna of Czestochowa, an ancient Catholic icon of the Virgin Mary believed to work miracles.
A pane of glass over the painting protected it from any damage during the incident on Sunday, local police spokeswoman Joanna Lazar said.
Guards at the monastery in the southern city of Czestochowa had quickly overpowered the man responsible and handed him over to police, she said.
The detained man had been questioned and was expected to undergo a psychiatric evaluation, Lazar said. His name has not been released.
Lazar said he could be charged with offending religious sentiment, an offence punishable by up to two years in prison.
"The faithful have been calling us crying to ask what happened," monastery spokesman Father Robert Jasiulewicz told the news agency PAP.
"We've reassured them that the painting is intact."
Many people in Poland, 90 per cent of whose population is Catholic, believe the icon holds special powers.
Every August, thousands of Poles make a spiritual journey to the monastery, a tradition that goes back to 1711 when bubonic plague decimated Warsaw's population.
When the epidemic suddenly ended, a brotherhood of knights trekked from the capital to the monastery to offer thanks to the Virgin Mary.
The relic has since enjoyed a strong following at home, and its international reputation was boosted by the late Polish Pope John Paul II who was deeply devoted to the icon.