European leaders moved early Thursday to stem the debt crisis gripping the continent by agreeing to a plan that imposes steep losses on investors holding troubled Greek bonds and boosts the firepower of the regions bailout fund to at least a trillion dollars. After marathon negotiations that continued well past midnight, European leaders said banks and other major investors in Greek bonds agreed to take losses of up to 50 percent. This concession was meant to help prevent the Greek government from defaulting on bills it cannot pay and avoid an even costlier shock to the European financial system.