The most natural is 'a police cordon'.
A cordon is a 'do not cross' line - which could be a line of people - but also could be a line of cones, or tape. If the police put it there - it is a 'police cordon'. 'A cordon of police' implies that there is a line of people, which would be a very inefficient use of personnel.
It may not be a common word in America, but it certainly is here, and is a formal/legal part of disaster management as well.