"The site of the Sisters of Our Lady of Mercy was completely blocked, and the attack was claimed by Islamist extremists using a server registered in Belgium," the sanctuary's spokesman, Sr Elzbieta Siepak, told Poland's Catholic information agency, KAI.
"Not all its elements are accessible again yet, although several thousand Internet users visit it daily in its seven language sections."
The Divine Mercy cult, which was banned by the Vatican between 1959 and 1978, is marked annually on the second Sunday after Easter and estimated to have 100 million followers worldwide via thousands of parish prayer groups.