A chief constable personally apologised after officers took four hours to respond to a call from student nurses who were being threatened by intruders.
Julie Spence, who heads Cambridgeshire police, said the response was "poor" and "inadequate for the situation" and also ordered an investigation into why it took officers so long to respond.
Student nurse Amy Overend and a friend made a call to police on August 3 after four men broke into their hospital accommodation building in Peterborough making threats to them.
The men continued shouting abuse outside the building and climbed up onto the roof, so the women made a second 999 call, said a force spokeswoman.
Police responded to the call at about midnight - more than four hours after their first emergency call - which was made at 8.17pm.
A Cambridgeshire police spokeswoman said: "The investigation will discover why the call was graded in such a way which did not merit a Grade A response."
Ms Overend told BBC Look East: "Obviously I was quite shocked at the fact that they only said it was a secondary emergency. It makes you wonder what is classed as a first degree emergency.
"We don't ring 999 for just anything - we ring them for a real reason so we thought if they think the situation is not that serious maybe we were making more of a to-do than anything else."Reuse content