The attack on the offices in downtown Srinagar, the state's summer capital, was followed by two more blasts and then a rattle of automatic gunfire, witnesses said.

Zee Television said 10 party workers were trapped in the building but police could not immediately confirm the report.

Security forces sealed off the busy Maulana Azad Road in central Srinagar where the offices are situated.

Police said at least one suicide attacker had entered the building, adding that senior Congress leaders were not present when the attack took place.

Kashmir police chief Gopal Sharma said "one or two" security forces members were hurt in the attack.

"There was firing and a grenade attack, one or two securitymen are injured," Sharma told Zee, adding that soldiers fired into the air to scare away the attacker or attackers.

"The area has been secured," he said, adding that the raid was aimed at disrupting elections here on Monday.

Other officials said two civilians were also injured.

Srinagar residents go to the polls on Monday to elect a candidate to represent their constituency in the staggered parliamentary elections that ends May 10.

The Congress party is a partner of the regional People's Democratic Party (PDP) which governs Indian-administered Kashmir.

On Thursday, two people were killed and six injured in two separate bomb attacks by militants in Srinagar city.

Separatist rebels have threatened Kashmiris with "unspecified consequences" if they participate in the elections that are being staggered to allow security forces and officials to move around the country.

Four people were killed and 25 wounded by rebels in voting in two constituencies in Kashmir Tuesday when the elections began.

A senior police officer said extra troops were being shifted to Srinagar and Budgam districts for Monday's polling.

Tens of thousands of people have died in the Himalayan region since the insurgency against Indian rule erupted in 1989.