The European Central Bank, after a hastily arranged meeting, signaled that it would invest in European bond markets in a bid to prop up hard-hit Italy and Spain, Europe’s fourth- and fifth-largest economies, which are in the midst of a worsening financial crisis.

U.S. and European officials are trying to calm anxiety about the global economy as the U.S. downgrade and European debt problems threaten to feed on each other, weighing on markets and a limp economy on both sides of the Atlantic.