"The hackers could penetrate deep inside the website and changed one paragraph," AICC Computer Department chief Vishwajeet Prithvijeet Singh said.
Singh said that Congress websites — aicc.org.in, congress.org.in — were subjected to the attack prompting the party to "put them down" this morning. The third website inc.org.in remained unaffected.
Singh, a former MP, said that the party websites have been under frequent attacks from hackers, who had some time back tried to break into the websites thrice one night.
The hacking came to light as the servers hosting the websites collapsed due to massive re-direction of internet traffic.
Singh said a detailed check of the servers has been carried out and the party will return to the cyberspace soon.
The cyber attack comes at a time when a controversy had broken out over Telecom Minister Kapil Sibal's comments on monitoring content on websites.
Sibal had talked about evolving guidelines and mechanisms to deal with "unacceptable" online content.
The defacement of Congress websites comes on a day when party chief turned 66.