By Ray Furlong
BBC Berlin correspondent
Tens of thousands of Roman Catholic pilgrims are pouring into the German city of Cologne for World Youth Day.
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The event reaches its high point on Thursday with the arrival of Pope Benedict XVI.
As many as 800,000 young Catholics from around the world will take part in a Mass given by the Pope on Sunday.
The city's hotels and guesthouses are filled to the brim and some 400,000 pilgrims are being put in private homes in the city and across the Rhineland.
They have been coming in by the busload from all over Europe - Ukraine, Poland, Romania, France - and by plane from across the world.
Cologne is awash with young Catholics bearing rucksacks and confused faces.
A force of 27,000 volunteers is helping them to their prearranged accommodation, but it is often chaotic work.
The streets are plastered with posters of the Pope, whose first foreign trip is - quite by chance - to his homeland.
The air of abnormality is heightened by a massive security operation in which the police have been sealing up manhole covers and will also close down the airport for the pontiff's arrival and departure.