"Our democratic institutions are under attack. Parliament is not being allowed to function and legislation is being bulldozed," Mr Singh told thousands of Congress activists at a 'Jan Akrosh' rally here.
He added: "Our democracy is in danger and we must all work together to protect it."
Mr Singh said PM Modi had not been able to fulfill a single promise made to the people four years ago before coming to power.
"Four years ago, a lot of promises were made to the people of the nation. But it is clear that they have not been able to meet even a single promise.
Slamming the BJP-led NDA government over the agrarian crisis, Mr Singh said: "The Modi government's anti-farmer policies have resulted in an unprecedented agrarian distress. Our farmers are demanding justice from the government.
The former prime minister said: "This government is anti-farmer. Farmers are feeling suppressed under farm loans and there are demands from every corner for farm loan waiver."
"Prices of fuel are rising in India despite low international crude oil prices. This price rise has greatly distressed the people."
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections. Election results will be out on May 23.