As of 0715 GMT, the continent-wide STOXX 600 index was down 0.5%, with the energy sector leading losses after a 2% drop. Most local bourses also opened in the red.
Oil prices dropped more than 3% on the prospects of a potential U.S.-Iran nuclear deal that could ease sanctions and boost supply. Major oil firms bore the brunt, with BP and Shell falling 5% and 3%, respectively, dragging down the main index.
Investors are awaiting remarks from Powell later in the day for potential insights into the monetary policy outlook. Attention is also focused on upcoming U.S. retail sales data and Walmart earnings, which could offer a clearer picture of consumer sentiment.
In the region, investors also awaited the release of the euro zone flash GDP and employment data for the first quarter, scheduled for later in the day.
Shares of Thyssenkrupp dropped 12% after the submarines-to-car parts group posted a plunge in its second-quarter operating profit.