What began as a weak day in the stock markets ended in the worst rout in more than two years, as investors dumped stocks amid anxiety that both Europe and the United States were failing to fix deepening economic problems.

With a steep decline of around 5 percent in the United States on Thursday, stocks have now fallen nearly 11 percent in two weeks. Markets have been plunging as investors sought safer havens for their money — including Treasury bonds, which some had been avoiding during the debate over extending the nation’s debt ceiling.

Sparking the drop was an unsuccessful effort by the European Central Bank to reassure the markets, which instead ended up spooking investors. The bank intervened with a show of support to buy bonds of some smaller countries, but not Italy and Spain, whose mounting troubles have come into the spotlight. This was taken as a sign that the recent rescue packages by Europe could soon be overwhelmed by the huge debt burdens in those two countries.

Investors were further unnerved by a candid remark by José Manuel Barroso, the European Commission president, who seemed to confirm fears about the sense of political paralysis. Rather than play down the problems, as European officials have done since the debt crisis began last year, he said, “Markets remain to be convinced that we are taking the appropriate steps to resolve the crisis.”