The incident happened in the early hours of this morning in Letterkenny.
Two men were injured when they were hit with a broken bottle, one of them suffering cuts to his face and arm.
Shocked eye-witnesses say the man appeared to be “going crazy” and was launching random attacks on innocent bystanders.
A number of people were forced to defend themselves from attack during the incident.
Gardaí attended the scene and a man was detained for questioning.
The incidents happened outside the Courtyard Shopping Centre.
The man launching the attacks appeared to be very drunk at the time.
Meanwhile in a separate incident, a man taken by ambulance from a nearby nightclub to Letterkenny General Hospital had suffered an epileptic fit.
He was later put into an induced coma but has since come round and is said to be in good spirits.