EDINBURGH — Once a year, the cobbled, medieval streets here host an extra million people, tripling the population of the Scottish capital for the nearly monthlong Edinburgh Fringe Festival, which used to be known as the biggest theater festival in the English-speaking world.

Now in its 70th year, the event has become the largest arts festival of any kind in the known world, topped in attendance, its organizers say, only by soccer’s World Cup or the Olympics.

The Fringe — which is taking place this month — began as a protest against the inaugural state-subsidized, highbrow Edinburgh International Festival after eight performers who were refused permission to perform there in 1947 started their own event. Many say it has stayed true to its anti-establishment roots. The theme this year, for example, is “Art as an act of defiance.”