Rupee was also down almost 96 paise, though the government and RBI sought to play a brave face saying the fall in Indian currency was relatively moderate in comparison to many other currencies globally.
RBI Governor Raghuram Rajan also promised full liquidity support to ensure orderly adjustments in the market, even though he maintained that Brexit referendum has caused a major adjustment in asset classes globally with gold and the US dollar going up and the stocks taking a plunge everywhere.
In the pre-open trade itself, the Sensex fell by over 634 points, while it opened sharply lower at 26,367.48, 940 points below its previous close, and plunged even further soon to touch a low of 25944.53 points with a massive decline of nearly 1058 points or about 4 per cent.
It recovered a small portion later to return above 26,000 level in late morning trade.
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Major losers were Tata Motors that dipped by 11.40 per cent, Tata Steel by about 9 per cent. ICICI Bank, Axis Bank, SBI, Adani Ports, Larsen, ONGC and Reliance Industries were also down significantly.
Foreign investors are said to have sold heavily, while some buying by domestic institutions at lower levels was seen to be of little help.
In overseas markets, all Asian markets tumbled as investors reacted nervously to results of the UK referendum.
Pre-open trends for the European and US markets also showed weak trends.
Concerns further mounted as commentators said Britain's exit, commonly referred to as Brexit, would mean that the EU could slip into recession while Indian companies would also need to rework their strategy to use UK as a gateway for their European operations.
The broader markets too displayed a firm trend as retail
investors widened positions, with BSE small-cap index rising 0.65 per cent and mid-cap 0.61 per cent.
Foreign funds sold shares worth a net Rs 0.19 crore yesterday, as per provisional data.
Asia and Europe, however, traded mixed as investors awaited further clues on whether the Federal Reserve will raise US interest rates this year, while oil suffered fresh losses on persistent glut worries.
Globally, Japan's Nikkei rose 0.61 per cent, while Hong Kong's Hang Seng shed 0.77 per cent and Shanghai composite lost 0.12 per cent.