MADRID — Spain raised $6.2 billion in a debt auction Thursday that saw strong demand and a drop in a benchmark interest rate, a sign that European plans to ease the debt crisis have helped investor confidence.

The Treasury sold $1.1 billion in benchmark 10-year bonds at an average rate of 5.67 percent, down from 6.65 percent in the last such auction Aug. 2.

It sold another $5.11 billion in three-year bonds at a rate of 3.84 percent, up from 3.6 percent.

The total raised was $388 million more than planned.

Spain’s borrowing costs have fallen from unsustainable highs since the European Central Bank in August unveiled plans to buy the bonds of financially weakened countries. The plan, along with financial aid from Europe’s bailout funds, would come with strings attached, however.

Madrid has said it may apply for the aid if the terms are reasonable but has so far not made an official request.

The economic crisis has left the country with near 25 percent unemployment and austerity measures and reforms aimed at reining in debt have triggered protests and sparked some regional tensions.