VATICAN CITY — The Knights of Malta, one of the most peculiar organizations in the world, is marking its 900th birthday with a colorful procession through St. Peter's Square, a Mass in the basilica and an audience with Pope Benedict XVI.

The Knights are at once a Catholic religious order, an aid group that runs soup kitchens, hospitals and ambulance services around the globe, and a sovereign entity that prints its own passports and enjoys diplomatic relations with 104 countries — yet has no country to call its own.

Some 4,000 people — volunteers and members, draped in their trademark black cloaks with a white, eight-pointed Maltese Cross on the front — processed through St. Peter's Square Saturday for the Mass marking the 900th anniversary of the order's recognition by the Holy See.