Police say at least six Hindu pilgrims, including five women, were killed and 13 others wounded when suspected militants opened fire on their passing bus in the Indian portion of Kashmir.
The annual yatra to the Amarnath cave shrine began from Jammu last week amid tight security.
"Thereafter, the militants again fired at a police patrol in which a passenger bus was hit by bullets".
In yet another tweet, the prime minister said, "India will never get bogged down by such cowardly attacks & the evil designs of hate".
Sonia Gandhi said that the attack was a crime against humanity and there should be probe into the lapses that led to it.
He tweeted, "Strongly condemn terror attack".
An official said it was not registered with the Shri Amarnath Shrine Board and was travelling against the advisory not to ply after sunset.
CPM leader Sitaram Yechury lashed out at the central government and posted series of tweets saying, "The situation in Kashmir has deteriorated under the BJP govt's watch".
The bus bearing the registration number GJ09Z 9976 was attacked at around 8.20 pm near Khanabal when it was on its way to Jammu, police said.
"This incident goes against the very grain of Kashmiri ethos", Syed Ali Geelani, Mirwaiz Umar Farooq and Mohammed Yasin Malik said in a joint statement in Srinagar, the main city of the Indian portion of Kashmir. The police also claimed that the bus driver had violated rules for the pilgrimage, which states that no yatra vehicle should be on a highway after 7 pm. "These are enemies of Islam, these are terrorists, and the attack should be investigated".
Soon after the attack, the Internet services in Kashmir were blocked on cell phones and broadband lines. "This is the first time yatris have been targeted".
Sources said there were general inputs regarding a threat to Amarnath yatris, but no specific input indicating the time or place of a possible attack.
The one thing we had all feared this year during the yatra. In spite of recent successes against militants & unprecedented force presence, ? he added.
In the attack, three Special Operation Group (SOG) were injured and another cop was also injured.