The Belgian policemen, from Ypres, were arrested and questioned before being released, an official source said. The policemen said they had picked up the migrants after finding them walking along a road in Belgium.
One of the officers, Georges Aeck, told Belgian broadcaster RTBF: "We didn't want to leave them... on the side of the road to walk to the border. "So we took them... in the direction they wanted to go."
Belgium's ambassador to France, Vincent Mertens de Wilmars, was summoned Thursday over the incident "so we can ask him for an explanation", the French interior ministry said.
French authorities have already expressed "their strongest condemnation after this initiative which does not conform to the normal work practices agreed between France and Belgium".
The Belgian police union said the French police had handcuffed their Belgian colleagues before questioning, a claim denied by the French authorities.
The migrants were taken to a border police station in the northern French city of Lille. Three minors were placed in the care of local authorities and the adults have been temporarily detained while their status is assessed.
The northern French port of Calais is a magnet for migrants who try to board lorries there to reach Britain, although it is not known whether the migrants in the incident had asked to be taken to Calais.
- with AFP