There have been a number of high-profile terror trials in Morocco
The Moroccan authorities say they have broken up a militant cell operating in several towns in the kingdom.
The group allegedly had links with al- Qaeda and police described the 15 people arrested as "dangerous".
They had electronic and chemical materials used to make explosives, reports the state press agency, MAP.
It is the fourth such network Morocco says it has broken up this year. An al-Qaeda-inspired group has said it was behind recent attacks in Algeria.
"The members of this structure, known as "Fath al-Andalous" [Conquest of Andalusia], planned to carry out attacks in Morocco and had formed operational links with foreign extremists who have pledged allegiance to the al-Qaeda organisation," MAP says.
The BBC's James Copnall in Morocco says this is almost certainly a reference to al-Qaeda in the Islamic Maghreb (QIM), an organisation largely based in neighbouring Algeria which has ties with Osama bin Laden's movement.
QIM says it has carried out attacks in several other North African countries, as well as the Algeria bombings which left 70 dead.
However, some analysts believe there is little real regional co-ordination between extremist groups.
Our correspondent says Morocco has been accused of over-dramatising the threat it faces from extremists, to benefit from support from the US and Europe.
Nevertheless, he notes that there have been attacks in Morocco: the most dramatic example occurred five years ago, when suicide bombers killed 45 people in Casablanca.