The demonstrators, who had arrived from all over the country, chanted "Erdogan terrorist" and "Freedom for Ocalan" as they carried Iraqi Kurdistan flags and portraits of Abdullah Ocalan, the detained leader of the Kurdistan Workers Party (PKK).
"The Europeans should hear us, empathize with our suffering and help us. It would be best if they imposed economic sanctions on Turkey," one of the demonstrators told Reuters.
The protesters said they wanted to reach out to voters in Turkey so that they vote against the constitutional changes that would increase presidential powers in the referendum.
Frankfurt police didn't interfere, saying on Twitter that the protest was peaceful and that most of the demonstrators had complied with German laws.
Turkey sharply criticized Germany for allowing the protest after placing a ban on referendum rallies by Turkish officials in German cities on security grounds.
Erdogan's spokesman Ibrahim Kalin said that by letting anti-Erdogan demonstrations take place, Germany was allowing open support for terrorism.
"We strongly condemn the German authorities for allowing the demonstrations by PKK terrorist supporters," Kalin said in a statement.
"We once again remind European countries: on April 16 the decision is to be made by the [Turkish people], not Europe."
Around 1.4 million out of 3 million ethnic Turks living in Germany are eligible to vote in the April referendum, according to Foreign Ministry spokesman Martin Schaefer.
Tensions between the countries escalated after Erdogan accused Germany of resorting to Nazi-era tactics in response to several German cities withdrawing their permission for the rallies in support of Turkish government.
