Calling the BJP-led government at the Centre "suppressor of freedom of speech", especially in the wake of demonetisation move, opposition leaders on Friday criticised the Centre for its authoritarian ways and called for maximum unity among the opposition.

Bihar Chief Minister and Janata Dal (United) leader Nitish Kumar appealed for maximum opposition unity after launching a book "Fearless in Opposition, Power and Accountability", authored by P. Chidambaram, former Finance and Home Minister and a senior Congress leader.

"Why should the opposition be afraid? It is opposition job's to put forth its views bravely. Nothing can happen in democracy without dialogue and debate. Opposition should get due respect. There should be maximum unity among opposition," Kumar said, adding that there was no need to get certificate of patriotism from the government.

The discussion on the book, which saw speakers such as Communist Party of India (Marxist) leader Sitaram Yechury and former union minister and Congress leader Kapil Sibal, focused on authoritarian ways of the BJP government and fallouts of demonetisation.

While Yechury called the methods of curtailing black money inappropriate, Sibal used the opportunity to call demonetisation "biggest scam ever".

The event was also attended by former Prime Minister Manmohan Singh and Congress Vice President Rahul Gandhi.

The speakers took a dig at Prime Minister Narendra Modi's for his "raincoat" comment against Manmohan Singh. Chidambaram called the comment "derogatory" while Yechury said: "It is not the level of politics."

Chidambaram said that strong exception must be taken to the comment since it was made on the floor of Parliament.

Chidambaram also criticised the BJP for making declaration of "Congress Mukt Bharat". He said there much was fear stalking the country, which demanded opposition to be fearless.

"If you criticise the government, you are anti-national, if you speak against the government, you are unpatriotic, if you dissent that is sedition. This is complete mockery of democracy. Institutions and individuals are in fear, Dalits are in fear, minorities are in fear," Chidambaram added.

"Without a united opposition, we can't have democracy. There are enough cheer-leaders, enough drum beaters (for the BJP government). We must be fearless, we need voice of the opposition," the former union minister said.

He called the demonetisation "a terrible mistake", which "damaged India's growth story". "We should wait for jury to give its verdict," he added.

Meanwhile, Kumar defended his decision to support demonetisation soon after it was implemented, saying "immediate opposition was not appropriate", but he said the government should now talk about the recovery of black money after over 90 days of the move.

"Now the Centre should tell us how we have benefited (from demonetisation). They are talking about cashless transactions now, diverting the topic. It makes no sense," Kumar said, adding that complete cashless economy was not possible in the country.

He further said Manmohan Singh's description of the demonetisation as "monumental management failure" was an apt one.

Yechuri said: "The popular perception is anti-black money and not demonetisation. There is no dispute that money, illicit money should be curtailed. But methods adopted are not good."

Yechury expressed hope that opposition unity would happen in future.

Sibal said that the country had lost space for informed debate. "The media is controlled by industry, corporate sector, which has its own agenda and nobody wants to go on the wrong side of the government. The bureaucracy is in fear. Even Supreme Court has failed us. We need to build consensus and to form new way of the way forward."

Sibal, however, evaded a question when asked about the Emergency days, saying: "The leadership has already said it publically. We should not repeat it. We should look at contemporary issues."