Mumbai, April 22: The bull rally suffered a sharp setback as aggressive selling by institutions and speculators sent share prices and indices tumbling down. The 30-share BSE sensitive index (Sensex) fell by 101.52 points in panic selling on the Bombay Stock Exchange.The market actually opened on a good note as Sensex touched 4,322 in the morning session, but tumbled by 152.56 points from this high level to close at 4,179.44. Calcutta, Delhi stock exchanges and the National Stock Exchange (NSE) also encountered selling pressure, mainly from institutions. UTI is believed to have unloaded shares worth Rs 150 crore.There was heavy unloading in software shares. Reports that stock exchanges will impose a special margin on software shares - which were shooting up in the last a few days - damaged the market sentiment. DSQ Software, Onward Technologies, CMC, HCL Infotech, Satyam Computers, Pentafour Software and NIIT which hogged the limelight during past five trading sessions on account of one way buy bidsfell flat on continuous bouts of sales. Several pharma shares also reacted on heavy selling pressure.Market circles said Wednesday's fall was a technical correction. Sensex had risen by nearly 500 points in the last one months and 1,000 points in the last three months. The rally was attributed to the installation of the BJP government at the centre, its assurance to foreign investors, market-friendly Export-Import policy and re-entry of foreign institutional investors.Bear operators were further emboldened by the fissures in the Vajpayee coalition government. ``The exit of Buta Singh and acrimonious exchanges among leaders like Jayalalitha, Ramakrishna Hegda and Ram Jethmalani. The Vajpayee government doesn't seem to be an exception. History is being repeated,'' said a BSE broker.``The market was overheated for a long time and today's correction which was inevitable, should be considered as a healthy sign for the market,'' said Ramesh S Damani, a BSE broker. The nervousness in the market constantlygrew over the rising differences in the cross exchange rates. The rising discrepancies in the prices of pivotals at the BSE, NSE and Calcutta exchanges with the badla rates zooming to unwarranted heights provided the base for today's correction, sources said.Institutional sales which kicked off in the form of profit booking unnerved the broking community who had taken huge exposure based on market perception in the absence of fundamental valuations. However, many scrips, especially MNC shares, staged a rally in the opening session.FIIs kept away from the market, thereby further unnerving local players.