Congress President Rahul Gandhi on Thursday termed the swearing-in of BJP leader B.S. Yeddyurappa as Karnataka Chief Minister a 'mockery' of Constitution after his party failed to garner a majority in the state assembly.

Yeddyurappa took oath as the Chief Minister in Bengaluru on Thursday hours after the Supreme Court declined to stay his swearing-in ceremony.

"The BJP's irrational insistence that it will form a government in Karnataka, even though it clearly does not have the numbers, is to make a mockery of our Constitution," Gandhi said on Twitter.

"This morning, while the Bharatiya Janata Party celebrates its hollow victory, India will mourn the defeat of democracy," he added.

Governor Vajubhai Vala administered the oath of office and secrecy to Yeddyurappa at 9 a.m. at the Raj Bhavan amid tight security.

Earlier, a three-judge bench of the apex court rejected a joint writ petition of the Congress and the Janata Dal-Secular (JD-S) to stall the swearing-in at a special pre-dawn hearing.

Although the top court said the swearing-in was subject to the final outcome of the matter before it and posted the case for further hearing at 10.30 a.m. on Friday.

It also sought the letter Yeddyurappa wrote to the Governor on Wednesday informing him his election as the Bharatiya Janata Party's legislature party leader in Karnataka.

On Thursday, Congress leader and former Finance Minister P.Chidambaram tweeted saying Yedyurappa's letter to the Governor will 'seal his fate'.

"Yedyurappa's letter to the Governor will seal his fate. There is no mention of a number bigger than 104. The Governor's invitation does not mention any number at all!" he wrote on the site.

"I salute the Supreme Court. If I were Yeddyurappa, I will not take oath until the hearing at 10.30 am on Friday," he added.