The BSE benchmark Sensex slumped to below the psychological 21,000 points mark in early trade on Monday as profit-booking wiped out initial gains and left the benchmark index over 137 points in the red.

The Sensex, which opened 71 points higher today, fell by 137.17 points, or 0.48 per cent, to 20,867.79 within the first 45 minutes of trading.

Similarly, the broad-based National Stock Exchange index Nifty lost 39.45 points, or 0.62 per cent, to 6,273.00 at the same time.

Stocks of banking, oil and gas, IT, realty and PSU sector stocks came under pressure on emergence of profit-booking, dragging the Sensex below the 21,000 points level, brokers said.