Indian stocks plunged deep into the red on Monday morning as the Sensex lost more than 500 points within minutes of the market opening on concerns over the US losing its top-notch credit rating due to mounting debts. The 30-share benchmark index dropped within minutes of opening to 16,793 points, its lowest level since June, 2010, registering a fall of 512 points from its previous close. The NSE's Nifty was also down by 131 points at 5,080 points at 9.30 PM, after hitting a low of 5,060 points earlier in the morning. The sharp fall of this morning came in the first trading session after the US lost its 'AAA' credit rating for the first time in history, as ratings agency S&P was not convinced with the efforts being made to tackle the country's debt problems. Today's fall follows an over ,000-point decline in the Sensex over the last four trading sessions amid weak global cues. All Asian markets were trading in the deep red on Monday morning, with huge losses seen in Hong Kong, South Korea and Japan. Hong Kong's Hang Seng Index was down 4.13 per cent, while Japan's Nikkei was trading 2.06 per cent lower this morning.