What role did the opposition parties play in maintaining the democratic character of the system?

They offered sustained and principled criticism of the Congress party.

The Rise of Opposition Parties in India:

India had numerous opposition parties during the period of the first few general elections after the independence, showcasing a greater diversity than many other multi-party democracies.
Some of these parties existed prior to the 1952 general election and played significant roles in the politics of the 1960s and 1970s.
Most of today's non-Congress parties can trace their roots back to the opposition parties of the 1950s.
Despite having limited representation in the Lok Sabha and state assemblies, these opposition parties played a crucial role in maintaining democracy.
They provided sustained criticism of Congress party policies.
They also kept the ruling party in check.
These opposition parties influenced power dynamics within Congress by influencing the factions within Congress.
These parties prevented anti-democratic sentiments by keeping democratic alternatives alive.
They helped in nurturing future leaders.