Key French and German equity indexes were down about 3 percent in midday trading.
Socialist challenger Francois Hollande won the first round of France's presidential election with 28.6 percent of the vote. Hollande has campaigned against austerity initiatives pushed by incumbent French President Nicolas Sarkozy.
Meanwhile, the statistical arm of the European Commission said the euro zone's overall debt increased to 87.2 percent of gross domestic product from 85.3 percent, a development that financial analysts found disconcerting.
We're in a debt crisis. Eurozone countries have way too much debt. We have gorged on debt. We are living beyond our means. And after 10 years of booming economic times, it is now payback time, Louise Cooper, a leading London financial analyst, told CBS News earlier this month.
Economic news was just as grim as the financial news. German manufacturing fell in April at its fastest pace in three years, and the euro zone recession is growing worse, Markit Economics reported.
The Markit Flash Eurozone PMI Composite Output Index tumbled to its lowest level in five months.
The German manufacturing PMI sank sharply to 46.3 in April from 48.4 in March, while the services PMI edged up to 52.6 from 52.1 in March.
The overall manufacturing performance of Europe's top economy was the weakest since mid-2009 as shrinking export demand reverberated through the sector. While firms reported resilient export sales to Asian emerging markets and clients in the United States, weakness in trade flows closer to home put the brakes on the manufacturing sector.
Meanwhile, lower workloads at German private sector companies contributed to a drop in unfinished business for the tenth consecutive month.
In afternoon trading, Germany's DAX plunged 3.36 percent, France's CAC-40 dropped 2.83 percent and Britain's FTSE 100 fell 1.85 percent.
In the U.S., the Dow Jones Industrial Average fell 137.36 to 12,891.90, the Nasdaq Composite lost 37.79 to 2,962.66 and the S&P 500-stock index gave up 14.19 to 1,364.34.