MTNL's corporate website could not be accessed, following the attack since afternoon and officials said efforts were underway to restore it.
However, its individual city-specific websites (for Delhi and Mumbai) were working.
MTNL Delhi, Deputy - GM (Internet), Deepak Sharma said it was not hacking but 'denial of service attack' under which the server is unable to provide services to the customers.
"The changes are being made and the website will be fully operational within the next 24 hours," Sharma told PTI.
A post on the Anonymous Operation India web page claimed responsibility for the hacking saying "we are against Internet Censorship. Instead of blocking few URLs, the ISP (MTNL) blocked the whole domain of various file sharing websites."
It has also asked netizens to wear 'Guy Fawkes' masks, which have become synonymous with Occupy Wall Street protests, during a fresh round of peaceful protests to be held on June 9.