Apparatus for building tyres, comprising: - a forming drum comprising an outer surface extending around a central axis and a rotating shaft extending along said central axis up to a free end; - a processing unit operatively active on said outer surface; - a movement unit for displacing said forming drum provided with at least one holding device comprising a fixed body; a rotatable body rotatably connected to said fixed body so as to rotate around a rotation axis corresponding, in use, to said central axis; a motor coupled to said rotatable body to drive it in rotation around said rotation axis; wherein said rotatable body comprises: - a first element extending along said rotation axis between a first axial end, rotatably connected to the fixed body, and a second axial end provided with a housing axial seat configured for receiving the rotating shaft of said forming drum, - a second element provided with at least one holding member for holding said rotating shaft, said second element being moveable with respect to the first element between a first axial position, in which it holds said rotating shaft, and a second axial position, in which it releases said rotating shaft, characterized in that said rotatable body comprises at least one elastic retention element operatively interposed between said first element and said second element and configured to keep the second element in said first axial position; wherein the holding device comprises an actuator operating between the first element and the second element and configured to oppose said at least one elastic retention element and displace said second element from the first axial position to the second axial position; and wherein the holding device comprises at least one anti-rotation unit selectively displaceable between an active condition, in which it rotatably constrains said rotating shaft of the forming drum and said first element of the rotatable body, and a passive condition, in which it does not interfere with said rotating shaft of the forming drum. - a forming drum comprising an outer surface extending around a central axis and a rotating shaft extending along said central axis up to a free end; - a processing unit operatively active on said outer surface; - a movement unit for displacing said forming drum provided with at least one holding device comprising a fixed body; a rotatable body rotatably connected to said fixed body so as to rotate around a rotation axis corresponding, in use, to said central axis; a motor coupled to said rotatable body to drive it in rotation around said rotation axis; a fixed body; a rotatable body rotatably connected to said fixed body so as to rotate around a rotation axis corresponding, in use, to said central axis; a motor coupled to said rotatable body to drive it in rotation around said rotation axis; - a first element extending along said rotation axis between a first axial end, rotatably connected to the fixed body, and a second axial end provided with a housing axial seat configured for receiving the rotating shaft of said forming drum, - a second element provided with at least one holding member for holding said rotating shaft, said second element being moveable with respect to the first element between a first axial position, in which it holds said rotating shaft, and a second axial position, in which it releases said rotating shaft, wherein the holding device comprises an actuator operating between the first element and the second element and configured to oppose said at least one elastic retention element and displace said second element from the first axial position to the second axial position; and wherein the holding device comprises at least one anti-rotation unit selectively displaceable between an active condition, in which it rotatably constrains said rotating shaft of the forming drum and said first element of the rotatable body, and a passive condition, in which it does not interfere with said rotating shaft of the forming drum.