The Sensex at the Bombay Stock Exchange benchmark fell 51 points, or 0.3 percent, to 18,671 within the first few minutes of trade today, as investors booked profits, amid weakness in other Asian bourses. Later, the Sensex swung between positive and negative territory, in volatile trade, to stand 25 points, or 0.1 percent in the positive zone, at 18,747 in afternoon deals, a short while ago.Other regional markets in Japan, Hong Kong, Singapore, South Korea and Taiwan were all in the red. China's key stock index tumbled almost 3 per cent, on concerns about a slowdown in that country's manufacturing sector.