CALAIS, France — French authorities began a complex operation Monday to shut down the makeshift camp known as “the jungle” in the French port city of Calais, uprooting thousands of migrants who made treacherous journeys to escape wars, dictators or grinding poverty and dreamed of building new lives in Britain.

Closely watched by more than 1,200 police, the first of hundreds of buses began transferring migrants to reception centers around France where they can apply for asylum. The camp will then be leveled in a weeklong operation.