Hurricane Sandy loomed Sunday over the northeastern U.S., triggering evacuation orders for hundreds of thousands of residents, the cancellation of nearly 8,000 airline flights, the mass closure of schools and public transit systems and the closure of U.S. stock and options markets, as authorities warned of heavy rains, high winds and flooding when the storm reaches land late Monday or early Tuesday.

U.S. stock and options markets will close Monday, exchanges and regulators said late Sunday night, the first unscheduled, market-wide...