The National Football League descended Friday into turmoil as negotiations between owners and players broke off, setting up a potentially lengthy court battle that could have serious consequences for both the league and the sports business in general.

After spending seven-plus hours negotiating in Washington, D.C., in a 17th day of mediation, leaders of the NFL and the players union said late Friday afternoon that talks had stalled and blamed the other side for the meltdown.

Shortly...