The Congress and many other parties on Thursday described Nitish Kumar's joining hands with the BJP as "treachery" with the people's mandate and said it was "a black day" for Indian democracy.
RJD leader Lalu Prashad dubbed Nitish as an "opportunist" and accused him of joining hands with the BJP to ensure the CBI and the ED raids on RJD leaders and his family members to malign him.
The CPI also said that Nitish could have waited for the court verdict on Tejashwi Yadav before taking the decision, while AAP accused him of adopting "double standards".
Hitting out at Nitish for joining hands with the BJP, Rahul Gandhi alleged he returned to the "communal forces" for his selfish political motives.
Rahul, whose party was part of the 'grand alliance' in Bihar that was deserted by Kumar's JD(U) last evening, said he was aware that it was being planned for the last three-four months.
"He (Nitish) had joined hands with us in the anti- communal fight, but Nitish Kumar, for his personal politics, has now joined hands with those against whom he had fought," said a seething Rahul.
This is the problem in India's politics, he added.
"People do anything for their selfish motives. There is no principle, no credibility. They can do anything for power," he told reporters when asked about the developments in Bihar.
Lalu also termed Kumar's "anti-BJP" drive as a well- scripted drama and the part of a "fixed game" between the JD (U) and the BJP.
Congress chief spokesperson Randeep Surjewala said Nitish had pledged never to join hands with the BJP and now his mask of "big talks and self professed honesty" stands exposed, with his sole principle being "power at any cost, power at all costs".
Recalling that Prime Minister Narendra Modi had questioned Nitish's "political DNA" ahead of the 2015 elections, he said, "It has been proved today that the political DNA of BJP and JD(U) is identical - opportunism, treachery, hunger for power and disregard of people's mandate."
"In Bihar, every political and democratic principle was murdered in broad daylight and a government of BJP-JD(U) has been formed today, which is neither people's mandate nor does it have the authority to rule. This is a black day indeed for Indian democracy," the Congress leader said.
Surjewala said it is an affront to the people's mandate and a "double whammy" for democracy that within 20 months, the politics of opportunism was repeated "when a party rejected by the people and a leader hungry for power at any cost formed a government overnight".
On opposition unity, he said, it is intact and all political parties are together.
Congress leader Mallikarjun Kharge termed as "unfortunate" Nitish joining hands with the "dictatorial" BJP after seeking votes in the name of "secularism" in the state.
"The grand alliance was formed to bring secular forces together, to fight the dictatorial attitude of the BJP, its attitude of killing democracy," he said.
Expressing shock over the developments in Bihar, CPI general secretary Suravaram Sudhakar Reddy said, "It's an unfortunate, negative development."
AAP leader Sanjay Singh said Nitish's move reflects his "double standards" and also asked, "Didn't Nitish realise the credentials of Lalu Prasad when he took his support after he walked out of the NDA, and then again at the time of Bihar polls. Now he suddenly finds Lalu Prasad corrupt." PTI TEAM SKC