Mumbai: The Bombay Stock Exchange benchmark Sensex fell by over 51 points in opening trade on Monday as funds and retail investors locked in gains recorded in the previous session amid a weak trend on other Asian bourses over US debt worries.

The cautious approach adopted by market participants ahead of Sensex heavyweight RIL's Q1 results announcement and the RBI's monetary policy review on July 26 cast a shadow on the trading sentiment, marketmen said.

The 30-share Sensex, which gained over 286 points in the previous session, fell by 51.46 points, or 0.27 percent, to 18,670.84 in the first few minutes of trade on Monday.

In a similar fashion, the wide-based National Stock Exchange Nifty index shed 17.25, or 0.30 percent, to 5,616.70 points.

Brokers said profit-booking by funds and retail investors of the handsome gains recorded in the previous session and a subdued trend on the other Asian bourses over US debt woes mainly dampened the trading sentiment.

In addition, fears that the RBI may announce another hike in interest rates at its policy review meeting on July 26 had a negative impact, they said.

Meanwhile, in the Asian region, Hong Kong's Hang Seng Index was down 0.90 percent, while Japan's Nikkei lost 0.88 percent in morning trade on Monday. The Dow Jones Industrial Average in the US ended 0.34 percent lower in the previous session on Friday.