French authorities have been scrambling to find shelter for 1,500 migrants left homeless after fire destroyed the Grande-Synthe camp near Dunkirk.
About 500 people have been accommodated in gymnasiums but the rest, including unaccompanied children as young as 12, have been sleeping outside, NGOs say.
Migration has been a key issue in the run-up to the poll on 23 April.
A spokesman for the centrist candidate, Emmanuel Macron, said an agreement with Britain that left the French acting “as its border guards” would have to be reviewed during Brexit negotiations.
His far-right rival Marine le Pen called a fight between Kurds and Afghans just before the blaze started as a sign of migratory chaos.
The Grande-Synthe camp was home to one of the largest groups of migrants attempting to reach the UK following last October’s destruction of the “Jungle” camp near Calais, about 40 km (25 miles) away.
The number of people there had almost doubled and the arrival of more Afghans increased tensions with Kurds living in the camp, reports said.
Five people were hurt in a knife fight between Afghans and Kurds which was broken up by riot police. Another migrant was in a serious condition after being hit by a vehicle on a nearby motorway, Reuters reported.
Sixty firefighters fought the blaze but it destroyed 70% of the densely packed wooden huts and left the site unusable.
The French authorities have said they will speed up admission to reception centres for migrants wanting to apply for asylum in France.
Campaigners have called on both the French and British governments to improve legal migration routes.
Last week, some Grand-Synthe residents tried to block a nearby highway with tree trunks and branches in an attempt to stop traffic and clamber onto trucks, Reuters reported.