Building on initial gains, the Sensex at the Bombay Stock Exchange had shot up 521 points, or 2.7 percent, to 19,791 in afternoon trade, a short while ago. Earlier in the morning, the Sensex had gained 401 points, or 2.1 per cent, to 19,671, on sustained buying by funds and retail investors, triggered by a strengthening rupee and a firm trend in the Asian region. The 30-share Sensex had gained over 1,000 points in the previous three trading sessions.Other Asian markets in Japan, China, Hong Kong, South Korea and Singapore had climbed between 0.6 percent and 1.5percent.