Sonia's selection appears to be a well-thought-out move as no Congress candidate has tasted defeat from this constituency since 1957. But the entry of Sushma Swaraj raises prospects of a battle royale as the firebrand BJP leader may prove a tough nut to crack.

Speaking to reporters soon after filing her nomination, Swaraj said she could not say no when the party leadership asked her to take on Sonia Gandhi. It was, she said, "my national duty."

Sonia filed four sets of papers at 2.20 pm, 40 minutes prior to the close of nominations, bringing to an end all speculation that she may make her maiden electoral foray from Cuddappah in neighbouring Andhra Pradesh.

Shortly after, the BJP brought in Swaraj to file her nomination, making it a high-voltage contest in the otherwise staid constituency which has returned Congress nominees since 1952.

Sonia was accompanied by party general secretary Ghulam Nabi Azad and Karnataka PCC president SM Krishna when she filed her papers before the returning officer Raj Kamal.

Swaraj made a dramatic appearance before the returning officer and submitted her papers. She was accompanied by party general-secretary M Venkaiah Naidu.

Congress had kept the choice of Bellary for Sonia a closely-guarded secret with even KC Kondaiah, who had been renominated as the party's candidate on Monday for the seat, claiming ignorance about it.

But the BJP appeared ready for the development and rushed Swaraj, considered be one of the staunchest critics of the Congress president, to Bellary to file her papers.

As news of Sonia Gandhi filing nomination from here broke, Congress workers went delirious with joy and burst crackers in front of the party headquarters in Bangalore.

Meanwhile, in Delhi, the Congress party was at pains to justify the suspense over party president Sonia Gandhi's constituency. Briefing mediapersons at the party headquarters, AICC general secretary Pranab Mukherjee had a tough time explaining why the name of the constituency from where Sonia Gandhi would contest was kept a secret. Mukherjee said, "There was no secret, but the party wanted some suspense". When asked if the suspense was created out of fear that early disclosure of Sonia Gandhi's seat would enable BJP to put up a strong candidate against her, party spokesperson Natwar Singh shot back: "Tell me, who is a bigger leader than Sonia Gandhi".

Mukherjee lost his cool when he had to face a volley of questions on why AICC general secretary Ghulam Nabi Azad announced on Tuesday that Sonia Gandhi would contest from Cuddapah in Andhra Pradesh,

On being queried about the nubmer of constituencies the Congress president would contest, he said Sonia Gandhi had filed her nomination from Bellary in Karnataka. "Fighting elections could not be kept a secret and as and when the nominations were filed people would come to know," he added.

Mukherjee evaded a question whether Sonia Gandhi would contest from any other constituency. Party sources, however, said that the announcement about Sonia Gandhi contesting from Amethi also, was expected any moment.