Also known as a Tonic-Clonic Seizure, formerly known as a grand mal seizure.

At the start of the seizure the person may cry out, then usually stiffens and falls.

Their arms and legs may jerk or twitch. You will not be able to rouse them (and do not try). Seizures generally last a few minutes, but can sometimes go on for much longer. During the seizure the person will often go blue in the face.  Try to remain calm.

DO:

1. Gently lay the person on their side. Make the person comfortable and put something soft under their head if you can. Clear a space around the person, moving objects away that might be harmful. Only move the person if they are in a dangerous place like by a fire, on the road or in water.
2. If there are other people around, explain what you are doing, reassure them if necessary and keep them away from the person having the seizure.
3. Loosen any tight neckwear and remove eye-wear and high heel shoes if necessary.
4. After the seizure talk to the person quietly, explain to them what happened and where they are. Stay with them until you are certain that they are okay.

DO NOT:

1. Do not put anything into their mouth at any stage.
2. Do not try to rouse the person.
3. Do not try to wake the person up; they will come-to in their own time.
4. Do not give them anything to drink until you are sure that they are fully awake.

AT THE END OF A SEIZURE:

Convulsing will stop, the person usually takes a deep breath, the blue color goes away and they slowly wake up. The person is often confused and will not know where they are for a short while afterwards. They may well be wet and soiled.
 

WHEN TO CALL AN AMBULANCE:

Be aware that you do not need to call for an ambulance unless one seizure runs into another, the person has not woken up after five minutes, they are having trouble breathing or they are injured.

http://www.irishhealth.com/clin/epilepsy/firstaid.html