Germany's highest court has opened a two-day hearing on a legal challenge to the European Central Bank's bond-buying programme unveiled last September to help struggling euro zone nations. The Federal Constitutional Court in Karlsruhe has to decide whether the scheme to buy sovereign bonds of debt-stricken nations from secondary markets, not been deployed yet but praised by many experts as a key factor for the current stability in the euro zone, violated the German constitution. More than 35,000 petitioners have argued that the ECB decision to buy sovereign bonds in unlimited quantities, if necessary, is in effect a direct financing of governments, which is banned under the EU rules. They complained that with its decision, the ECB crossed the limits of its mandate, which is to ensure price stability in the euro zone. The petitioners also expressed concern that the scheme, known as Outright Monetary Transactions, OMT introduced by ECB President Mario Draghi, exposed taxpayers in Germany and other euro zone nations to unforeseen risks over which officials have no control. German Finance Minister Wolfgang Schaeuble, who represented the government at yesterday's hearing, defended the ECB's policies to shore up the cash-strapped nations and denied that the bond-buying programme violated its mandate.