The BSE Sensex dropped 4.8 per cent at the open on Thursday to its lowest in more than 2 years after another sell-off in US and Asian markets on fears of a severe global downturn.Reliance Industries and ICICI Bank were leading the losses.At 9:58 a.m., the 30-share BSE index had pared some losses to be down 3.81 per cent at 9,782.36, but all its components were in red.The index was trading at its lowest level since June 2006.The 50-issue NSE index fell 4.04 per cent to 2,941.20.The index is down by half in 2008, with foreigners selling a net of $12.2 billion in stocks so far this year, largely reversing 2007's record net buying of $17.4 billion.Leading private sector firm Reliance Industries, which has the largest weighting in the main index, is forecast to report a modest 2.2 per cent rise in net profit on slimmer refining margins. Japan's Nikkei and MSCI's measure of other Asia-Pacific stocks were both down 5.5 per cent.US stocks tumbled to five-year lows on Wednesday with investors grappled with an increasingly dire outlook for the global economy following a raft of disappointing profits and outlooks from major US companies.