In an uncharacteristic hard-hitting speech at a convention here, Singh said institutions of democracy were "under threat" and the edifice of welfare state was being "dismantled".
He also questioned BJP government's claims on economic development, saying rural India was under acute distress and the economic recovery was fragile.
"Institutions of Democracy are under threat. The entire edifice of the welfare state is now being dismantled in the guise of promoting faster economic growth, whereas they are two sides of the same coin.
The Congress party, he said, has tried hard to ensure that India flourishes as an open pluralistic, liberal and secular democracy which was proud of its composite heritage.
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"This very idea of India is now under systematic assault. We must understand how this assault is being mounted and then come up with our response," he said.
A renowned economist, Singh also raised the issue of government's performance on economy to attack it, saying that even government's own people felt that the recovery is "fragile".
He said the Chief Economic Advisor of the Minister of Finance and the Governor of the Reserve Bank, have both been of the view that "recovery is fragile".
"And indeed if you look at the harsh reality you will find all is not well with the state of our economy under the BJP dispensation," he added.