Hundreds of thousands of protesters have taken to the streets of Prague demanding that PM Andrej Babis resign.
Mr Babis is facing a criminal investigation over alleged fraud, and is also the subject of an EU conflict-of-interest investigation. He denies the allegations.
Organisers say about 250,000 people attended the rally. The figure could not be verified.
It is said to be the largest protest since the fall of communism in 1989.
The protest was organised by the Million Moments for Democracy group. The movement has grown from a small student initiative into a mass movement.
It has organised a number of protests this year. The last major rally was on 5 June when 120,000 people marched in the capital.
The prime minister is facing a criminal investigation in the Czech Republic for alleged abuse of €2m in EU subsidies over 10 years ago.
Police say they are investigating accusations that Mr Babis hid his ownership of a farm and conference centre known as “Stork Nest” (Capi hnizdo in Czech) so that it could qualify for an EU subsidy designed to help small businesses.
The billionaire is also the subject of an EU conflict-of-interest investigation over his Agrofert conglomerate, which he transferred to two trust funds shortly before he came to power in 2017.