Congress leader P Chidambaram also alleged that the Governor has given Yeddyurappa 15 days’ time to "convert" the number of 104 legislators to 111.
"Governor gives Mr Yeddyurappa 15 days to convert the number 104 into 111," he tweeted.
Party general secretary Ashok Gehlot said that the decision was a "murder of democracy" and has been taken "under pressure from the BJP leadership".
"It is a murder of democracy in the country and it is a black day in India's history...The decision has been taken under pressure of BJP leaders," Gehlot told PTI.
Gehlot expressed concerns over the development and said people of the country are "worried" over the direction in which the country was moving.
In a communique, Vala invited Yeddyurappa to form the government and take oath as chief minister tomorrow. He also asked Yeddyurappa to seek a vote of confidence within 15 days of assuming office.
The BJP has emerged as the single largest party in the southern state winning 104 seats, but is short of a simple majority. On the other hand, the Congress and JD(S), which have already announced post-poll tie-up, have won 78 and 37 seats respectively and claimed before Vala to have numbers to form government in the state.
Congress' chief spokesperson Randeep Singh Surjewala also hit out at Vala, accusing him of denigrating the Governor's office, trampling upon Constitution and abusing law and acting like a "BJP puppet".
Vala, Surjewala charged, chose to serve "his masters in the BJP" than serving the Constitution.
"Vajubhai Vala denigrates the Governor's office, tramples upon Constitution, abuses the law & acts as a BJP puppet. He chooses to serve his Master's in BJP rather than serve the Constitution. As @BJPKarnataka informed in advance, orders come from BJP HQ then the sanctity of office," he said on Twitter.