PRAGUE — In the square in the heart of Prague, where crowds gathered three decades ago in their bid to wrest freedom from Communist rule and where independence was proclaimed seven decades before that, protest songs rang out again on Tuesday night.

Tens of thousands of men, women and children, coming from across the Czech Republic, waving flags and carrying signs attacking the government, gathered for what they said was yet another struggle for the soul of their democracy.

What started six weeks ago as a relatively contained protest — over the appointment of a justice minister many believe will protect Prime Minister Andrej Babis from potential fraud charges — has grown into something broader and possibly harder to control. Organizers said Tuesday that as many as 120,000 people had attended the protest, a count that would make it one of Prague’s largest demonstrations since 1989.

“This is about more than just corruption,” said Tomas Peszynski, 44, holding the corner of an oversize European Union flag. “This is about an abuse of the system of government and a fight to protect the institutions of democracy.”