The vessel was detained in the port of Palermo for 11 hours for an inspection the NGO Seawatch International called "arbitrary."
It comes after fire destroyed the Moria migrant camp, which was home to more than 12,500 migrants. Its capacity was 2,700 people.
The Mediterranea NGO said however that Malta has refused them access to a port to disembark the migrants.
Dozens of people protested outside a migrant detention centre in the French city of Marseille on Saturday, insisting those inside were at risk of catching COVID-19
As of Aug. 31, the Moria camp on the island of Lesbos housed 12,714 people, well over its capacity of 2,757.
Five years ago, the picture of three-year-old Alan Kurdi washed up dead on a Turkish beach shocked the world. Nevertheless, refugee and migrant children are "often worse off" now than in 2015, says Save the Children.