Colin Below, a spokesman for "Occupy Frankfurt", said about 30 tents had been pitched outside the ECB building and that the protest - originally given official permission until Wednesday - had now been extended "indefinitely."
The demonstrations will continue "for as long as necessary and possible," he said.
Meanwhile, police in Berlin, where an estimated 10,000 people took part in protests on Saturday, said they had successfully prevented demonstrators setting up camp in front of the Reichstag parliament building overnight.
There were a handful of arrests and some officers were injured in minor scuffles, a police spokeswoman said.
A fresh demonstration was planned later Sunday in front of the city's iconic Brandenburg Gate, authorities said.
Max Bank from anti-globalization movement Attac said the protest wave was only just beginning.
The demonstrations were "a clear sign that people want more democracy," Bank told German radio station MDR Info.
"The power of the banks must be broken. People no longer want to be dictated to by the financial markets and politicians must realise this," added Bank. "We have seen the beginning of the movement. That does not mean that we will not see more in the coming weeks."
Attac said that an estimated 40,000 people hit the streets in Germany as part of a global day of protest that saw rallies in 951 cities in 80 countries and violent clashes at demonstrations in New York and Rome.