The mass rally, organised by leftist opposition parties, urged Hungary’s populist government to respect democratic rights.
Mr Orban has been widely accused of undermining the rule of law and cracking down on press freedom since taking power in 2010.
The European Parliament last week voted to trigger disciplinary action against Hungary for flouting the EU’s key principles.
Demonstrators at today’s rally waved both Hungarian and EU flags, and carried placards bearing slogans such as “Orban out!”
The protest organisers wrote on Facebook: “Europe stood by us, now it is our turn. Orban and his Fidesz party lost in the European Parliament, and Hungary has won.”
The European Union‘s 28 leaders, including Mr Orban, will meet at a high-profile summit in Salzburg, Austria on Wednesday.
But Brussels sources have said the Hungary issue is unlikely to be raised, as the talks will focus on migration and Brexit instead.
One EU diplomat told the Financial Times: “Orban will surely be around the table, but at this point, he’s not on the table.”
With Brexit just six months away, the row over Hungary highlights tensions between the EU’s nationalist and federalist camps.
Mr Orban’s party vowed to challenge last week’s European Parliament vote, saying the MEPs’ motion was “petty revenge”.
Poland and the Czech Republic have both vowed to back Mr Orban and veto any sanctions against the Hungarian government.
Czech Prime Minister Andrej Babis said: “This nonsense just ushers in negative sentiment into the European Union.
“Instead of getting together, there is politics. So I stand behind Prime Minister Orban. We are allies.”
Poland’s Foreign Ministry added: “Every EU member state has a sovereign right to make internal reforms it deems appropriate.
“Poland will vote against any possible sanctions to be imposed on Hungary within the European institutions.”
Mr Orban was re-elected in April after winning a third successive election, leaving Hungary’s left-wing opposition in disarray.