A man (33) is fighting for life in hospital today after being attacked in the centre of Derry.
Two men arrested in connection with the assault were being questioned by detectives.
Police were examining two potential crime scenes, one at John Street near the Craigavon Bridge and the other at Bayview Terrace close to the Strand Road police station.
Another man is being treated in hospital in Coleraine for lacerations to his back after what police described as an altercation with a number of youths in the Circular Road area late last night.
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A man and a woman were arrested for questioning about the attack.
Elsewhere, a number of petrol bombs were thrown at St Patrick's primary school in Garvagh, Co Derry, early today. Damage was limited.