"An investigation is under way to establish the exact causes and circumstances of the incident," the interior ministry said in a statement on Sunday.
'Music for modernity'
The concert was part of a week-long annual Mawazine music festival - one of a growing number of well-funded music gatherings aimed at promoting an image of tolerance and modernity in the north African kingdom.
Tens of thousands of fans have flocked to the festival featuring Kylie Minogue, Ennio Morricone, Khaled, Alicia Keys and Algerian diva Warda al Jazairia.
Mawazine, which is backed by the country's King Mohammed VI, aims to liven up the capital Rabat and ward off the growing cultural influence of opposition Islamic groups, observers say.
Some opposition politicians have denounced the pop concerts as encouraging immoral behaviour and say it would be better to spend the money on creating jobs.