Mumbai: Sensex fell further by over 75 points to 18,134.42 in opening trade on Friday, extending its losses for the fourth straight day on continued selling by foreign funds and retail investors, amid weak global cues.

The 30-share Bombay Stock Exchange Barometer, which lost over 661 points in the previous three sessions, moved down further by 75.10 points, or 0.41 percent, to 18,134.42 in the first few minutes of trade on Friday, with stocks of metal, realty and banking companies remaining under pressure.

Similarly, the wide-based National Stock Exchange index Nifty shed 26.30 points, or 0.48 percent, to 5,461.45.

Market experts said trading sentiments remained bearish amid a weak trend on other Asian bourses, tracking overnight losses in the US market on debt crisis and weak economic data.

Also, fears of more monetary tightening by the Reserve Bank of India also cast its shadow on the sentiments, they said.

In the Asian region, Hong Kong's Hang Seng index was trading 0.05 percent and Japan's Nikkei shed 0.35 percent on Friday morning. In the US, the Dow Jones Industrial Average ended 0.51 percent lower in Thursday's trade.