Oil refiners and retailers were among the worst hit in today's session.

The S&P BSE Sensex and NSE Nifty 50 indexes staged their biggest single-day fall since September 3 led by declines in HDFC, Reliance Industries, State Bank of India, HDFC Bank, Larsen & Toubro and IndusInd Bank. The benchmarks opened lower and extended declines in afternoon trading as oil & gas, banking and financial services shares came under selling pressure. In the intraday deals, Sensex fell as much as 356 points and the Nifty 50 index fell below important psychological level of 11,000 to touch an intraday low of 10,968.20.

The Sensex fell 262 points or 0.7 per cent to close at 37,123 and the Nifty 50 index dropped 72 points or 0.65 per cent to shut shop at 11,004.

Thirteen of 19 sector gauges compiled by BSE ended lower led by the S&P BSE Oil & Gas index's 1.6 per cent decline. S&P BSE BSE Bankex, Auto, Capital Goods, Realty, Finance and Energy indexes also dropped between 0.7-1.2 per cent.

On the other hand, S&P BSE Consumer Durables index was top gainer, up 1.3 per cent.

The mid- and small-cap shares ended mixed but outperformed their larger peers. The S&P BSE MidCap index fell 0.4 per cent while the S&P BSE SmallCap index rose 0.6 per cent.

Oil refiners and retailers were among the worst hit in today's session after a drone attack on Saudi Aramco's oil facilities sent crude prices surging nearly 20 per cent intraday, its biggest single-day gain since 1991.

Shares in state-run oil companies Bharat Petroleum, Hindustan Petroleum and Indian Oil fell as much as 6.97 per cent, 7.31 per cent and 4.08 per cent respectively. Private sector refiner Reliance Industries also fell 3.02 per cent.

Mahindra & Mahindra, State Bank of India, UPL, Indiabulls Housing Finance, Yes Bank, Asian Paints, HDFC and Tata Steel were among the losers in the Nifty 50 basket of shares.

On the flipside, Titan, Britannia Industries, Tech Mahindra, ONGC, Coal India, Sun Pharma and Wipro were among the gainers.