The present invention provides novel anti-factor XI (FXI) antibodies and compositions comprising such antibodies. The anti-FXI antibodies of the invention specifically bind to the active center of FXI and inhibit the functional activity of FXI. The invention further provides humanized versions of the anti-FXI antibodies that are useful in the prevention and treatment of conditions in which pathological thrombus formation or thrombo-embolism are involved. The invention further provides nucleic acid molecules encoding the anti-FXI antibodies, cells expressing the anti-FXI antibodies and methods for producing the anti-FXI antibodies.