Aircraft known or believed to be in a state of emergency, including being subjected to unlawful interference, shall be given priority over other aircraft.

The incident/accident report shall be completed and submitted by Controller on duty in accordance with the existing form to his Superior as soon possible.

Responsibility
ATS personnel shall be prepared to recognize any indication of the occurence of emergency. ATS personnel must always bear in mind the possibility of an emergency situation.

^^Distress signal from an aircraft may be madeby one or more of the following ways:
 a. the display os SSR Mode A Code 7700
 b. the signal "SOS" with signaling apparatus;or
 c. transmission of distress message "MAYDAY, MAYDAY, MAYDAY"

^^Urgency signal from an aircraft may be made by oneor more of the following ways:
 a. a succession of white pyrotechnical light fired at short interval;
 b. transmission of distress message "PAN, PAN, PAN"

^^Bomb threats
A bomb threats involving an aircraft is to be treated as an emergency. Any information or warning that the safety of an aircraft is endangered by a timed-bomb or explosives carried on board an aircraft calls for immediate ATC action. ATC shall regard bomb threats as genuine. Appropriate safety measures shall be taken until it can be reasonably assumed that the bomb-call is a hoax.

source by Air Traffic Control Training Book