U.S. stocks closed about 1.5 percent lower or more Monday, extending Friday's post-Brexit sell-off with materials leading decliners. The Dow Jones industrial average and S&P 500 ended at their lowest since mid-March.

The major indexes came off session lows as the close neared. Traders said there were more buy orders at the close when some had expected to see selling. They also said the market was lifted by investors buying stock to cover short positions.