German Foreign Minister Annalena Baerbock. Twitter
Baerbock told reporters that Berlin viewed Ghannouchi's arrest "with the greatest concern" and cautioned that the "democratic achievements in Tunisia since 2011 must not be lost".
Ghannouchi, 81, a former speaker of parliament, was arrested on Monday after remarks warning that eradicating different viewpoints such as the left or political Islam, from which his party originated, might lead to a "civil war".
The main opposition alliance, the National Salvation Front (FSN), of which his Ennahdha party is a member, said he had been held on suspicion of "plotting against state security".
Tunisia is heavily indebted and facing high inflation and unemployment, leading some of its citizens to try fleeing to Europe.
Since early February, authorities in the North African country have arrested more than 20 political opponents and personalities.
The Islamist-inspired Ennahdha party held the most seats in Tunisia's parliament before President Kais Saied dissolved the chamber in July 2021 in a power grab allowing him to rule by decree.
Saied, 65, claims those detained were "terrorists" involved in a "conspiracy against state security".
Opponents have dubbed his actions a return to autocratic rule in the only democracy to emerge from the Arab Spring uprisings in the region more than a decade ago.
Tunisia has been negotiating for several months with the International Monetary Fund (IMF) for a loan worth nearly $2 billion, but discussions appear to have stalled.
Baerbock acknowledged the country was "confronted with a difficult economic and social situation" and said the European Union did not want to "leave the Tunisian people alone" to grapple with it.
She said it was "imperative" that the Tunisian government do its part to ensure a successful end to talks with the IMF.