Between 40,000 and 50,000 protesters filled the square demanding the socialist Prime Minister's resignation.
There's been widespread anger since the leaking of a tape-recording on which Mr Gyurcsany admitted he lied to the electorate to win re-election in April.
So far there has been no repeat of the violence that marred earlier rallies and organisers have been handing out flowers and asking people to remain peaceful.
Protest organisers say they will announce the start of a campaign of civil disobedience.
The turmoil in Hungary coincides with a political shake-up in Poland, the failure to form a new government in the Czech Republic, raising concerns about political instability in central Europe.