Police were called after reports of the incident in Chandler’s Ford just after 10.30pm on Saturday at an address in Park Road.
All five of the boys, three aged 16 and two aged 17, were taken to hospital having suffered serious injuries.
Officers established that an unknown group had gained entry to the party.
Police remain at the scene as enquiries continue on Sunday.
Senior investigating officer DCI Elizabeth Pirie said: “We are already hard at work identifying anyone who was at the party who may be able to assist our enquiries, whether that be having directly witnessed what took place, having filmed any footage which may be relevant, or who may be able to help us identify the unknown group.
“I understand the response the local community may have to hearing about this incident this morning, and I do not underestimate the impact our scenes will be having on those living in the area. These scenes are vitally important for our enquiries and we are grateful to the understanding of our communities whilst we work to establish the facts.
“If you do have any concerns or information, then please do not hesitate to speak to us.
“We particularly want to hear from anyone who can provide information to support our investigation. If you were at the party and have not spoken with officers yet, or you have any CCTV, mobile phone, dash cam or doorbell camera footage covering Park Road and the surrounding area, then please get in touch.
“Any information, however small it may seem, could help our enquiries.”
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