Nadia Yassine, daughter of the spiritual leader of Morocco's leading opponent to the North African monarchy, faces several years in prison and heavy fines if found guilty.
The case against Yassine has raised concerns abroad about the freedom of expression and the press in Morocco, which has pledged to ease restrictions.
A Rabat court postponed the trial to hear two journalists as witnesses but did not set a new date.
Police dispersed hundreds of Islamists who protested in front of the court, chanting: "Enough is enough." Analysts say a conviction could anger the non-violent Islamist movement Justice and Charity.
Yassine told a newspaper earlier this month she expected the monarchy to collapse soon. "Moroccans can live without King Mohammed," she said. She called for the setting up of a republic.
But she has said she was only expressing her personal views.
Yassine arrived in court wearing band-aid over her mouth to protest against what she calls an attack on freedom of expression.
On trial with her is also Adelaziz Gougass, editor of the al-Ousbouaaya al-Jadida daily that published the interview. His case was also postponed.
Yassine has headed the movement since her father Abdeslam was jailed under late King Hassan, the father of the current king.
King Mohammed VI is under
pressure to liberalise the country
Justice and Charity has a strong following in universities and is popular in poor areas. It is banned from politics but allowed to do charity and other work linked mainly to education.
However, it has flexed its muscles several times in the past by rallying up to hundreds of thousands of sympathisers in demonstrations mainly in solidarity with the Palestinians.