(AP) Pope Benedict XVI thanked the Swiss Guards on Sunday for their 500 years of service protecting the popes, as the Vatican opened its official commemorations of the anniversary of the first mercenaries' arrival from Switzerland.

An honor corps of Swiss Guards, in their full blue- and-yellow bloomered dress uniforms and red-plumed helmets, stood at attention in St. Peter's Square as Benedict addressed them from his studio window. He recalled how Pope Julius II had summoned the mercenaries to protect him and the Vatican. They arrived Jan. 22, 1506.

"Thank you for your service of 500 years!" Benedict told the guards in a special blessing, to applause from tourists and the faithful also gathered for his traditional Sunday greeting.

More senior guards in red velvet barked out orders as the few dozen guards marched in lock step to the tunes of a marching band and the tolling bells of St. Peter's Basilica.

Earlier in the day, Benedict's No. 2, Cardinal Angelo Sodano, celebrated an intimate Mass in the Sistine Chapel for members of the current corps and their families _ the official start of the Vatican's commemorations for the half-millennium the Swiss have been protecting popes.

In his homily, Sodano recalled that Julius had summoned the Swiss because they had proven themselves "defenders of the freedom of the church."

"And thus started the long series of generous and strong young men who wanted to come to defend the Cathedral of Peter," Sodano said.

The Swiss Guards, known for their traditional halberds, have since protected 42 popes. Some are armed with guns.

Over the next six months, Vatican and Swiss authorities will host ceremonies, concerts, exhibits and commemorations that will culminate with a symbolic re-enactment of the march from Switzerland to Rome of the first 150 Swiss mercenaries.