Sensex dropped 651 points of 3.45 percent to close at 18,234.Earlier in the morning, however, the Sensex had gained 121 points, or 0.6 per cent, to 19,007, amid firm regional bourses. The Sensex had rallied almost 920 points in the previous four trading sessions.Stock markets in Japan, China, Hong Kong, Singapore, South Korea, Indonesia, and Taiwan had added between 0.1 percent and 3 percent, today, on data indicating a pickup in global manufacturing.