x4 images. Double click to enlarge.
These extraordinary growths are caused by a miscroscopic mite called Eriophyes tiliae, and it is commonly known as the Red Nail Gall Mite.
They appear on the upper surface of the leaves of Large-leaved Lime (Tilia platyphyllos) and the hybrid Common Lime(Tilia x europaea). I found this tree in a local park covered in them.