Stocks extended losses on Thursday, with the S&P 500 hitting session lows as traders cited anxiety over Saudi police firing on protesters.

This last leg down is definitely after a report saying the Saudi police are firing on protesters, said Robert Francello, head of equity trading for Apex Capital in San Francisco.

Saudi police dispersed a protest by a Shi'ite minority in the OPEC member's oil-producing Eastern province on Thursday, witnesses said. Saudi Arabia is the world's top oil exporter.

Nearly six stocks fell for every one that rose on both the New York Stock Exchange and the Nasdaq, showing the depth of the sell-off.

The benchmark S&P 500 index fell below its 50-day moving average -- an indication of medium-term momentum for the market -- for the first time since November.

The Dow Jones industrial average <.DJI> tumbled 212.59 points, or 1.74 percent, at 12,000.50. The Standard & Poor's 500 Index <.SPX> dropped 23.25 points, or 1.76 percent, at 1,296.77. The Nasdaq Composite Index <.IXIC> slid 47.15 points, or 1.71 percent, at 2,704.57.

(Editing by Jeffrey Benkoe)