Hungary has sealed its border with Serbia.
A razor wire fence was moved into position around 1am CET. A railway line, also used as a crossing point, has also been blocked off.
Hungarian government spokeman said, "Today we have witnessed an all time peak in in terms of illegal border crossing. The real number is going to be close to 10,000 in just one day."
Hungary, much critisised for its attitude to asylum seekers, is to introduce tougher measures later on Tuesday including plans to arrest illegal immigrants.
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