The website of a key investigative agency of the Indian government was hacked and defaced late Friday, according to a government official.
The website of the Central Bureau of Investigation was not accessible even by late Saturday.
The hackers calling themselves the "Pakistani Cyber Army" left a message saying that the attack was in revenge for similar Indian attacks on Pakistani sites, according to local media reports.
The border dispute between India and Pakistan over Kashmir has often spilled online, with both sides attempting to hack each other's sites.
The website of Pakistan's Oil & Gas Regulatory Authority was hacked on Saturday by a group called "Indian Cyber Army" in retaliation for the CBI website hack, according to The Express Tribune newspaper in Pakistan. The website displayed the message "This Account has been suspended" late Saturday.
Some other sites in India were also defaced, according to various reports.
But the defacement of the CBI site is likely to be the more embarrassing for the Indian government because it is a top investigative and counter-crime agency of the government.