Tunisia on Tuesday denounced "American interference" in judicial affairs after the US ambassador criticised a ruling that fined a television station boss for showing a film that depicted God.

The US ambassador in Tunis Gordon Gray last Thursday expressed "serious concerns" after a court fined the Nessma station's chief executive Nabil Karoui for broadcasting the Franco-Iranian film "Persepolis".

"The declarations of the American ambassador to Tunisia constitute interference in Tunisian justice," the foreign ministry announced in a statement reported by the official TAP news agency.

"The Tunisian government declares itself to be deeply astonished" by these statements.

The film, which looks at the Iranian revolution through the eyes of a little girl, features a controversial scene showing a depiction of God. Sunni Muslims consider portrayals of Allah to be blasphemous.

Karoui was on May 3 fined 2,400 dinars (1,300 euros, $1,700) in a high-profile trial on conviction of "broadcasting a film that disturbs public order and threatens proper morals."

Gray then issued a statement saying that the verdict "raises serious concerns about tolerance and freedom of expression in the new Tunisia."

Tunisia is led by the moderate Islamist Ennahda party that won the most seats in October elections, the first since the January 2011 toppling of long-serving dictator Zine el Abidine Ben Ali in an uprising that sparked the Arab Spring.

Some have voiced concern that Ennahda's rise constitutes a setback for secular values in Tunisia, although the party, which does not have a legislative majority, has promised moderation.

In his statement after the ruling, Gray said free speech was "a fundamental human right denied Tunisians during the Ben Ali era."