Spanish bond yields surged the most this month as a second night of violent protests loomed amid sparring over the police response to clashes in Madrid.

Spain's 10-year benchmark yield rose above 6 percent, approaching the levels seen before European Central Bank President Mario Draghi offered to buy struggling nations' debt. Prime Minister Mariano Rajoy told the Wall Street Journal in comments confirmed by his office that he would "100 percent" seek a rescue if borrowing costs stayed "too high."

Rajoy's efforts to restore investor confidence suffered a new setback yesterday when Catalan President Artur Mas called early elections to push for "self-determination" for the country's largest region. The move adds a new front to Rajoy's battles as he seeks to persuade voters to accept the deepest budget cuts on record.