The machete fight happened shortly after 6.30pm yesterday (Wednesday).
A 15-year-old boy suffered a fractured skull after he was struck on the head with a machete during the fight in the ginnel that runs between York Road and Rookwood Avenue, Osmondthorpe.
He also suffered a wound to his leg.
A 17-year-old boy was also seriously injured during the fight.
He received a wound to his wrist.
Their injuries are not life-threatening and they remain in hospital.
Both boys have been arrested on suspicion of affray.
A 16-year-old boy, who was carrying a machete and some cannabis, was also detained nearby by police.
He was arrested on suspicion of two offences of wounding with intent and possession of drugs.
He remains in custody.
Detective Inspector Guy Shackleton, of Leeds District CID, said: “The younger male has received a very significant head injury consistent with being struck over the head with a machete, and we are treating this incident very seriously.
“We are carrying out extensive enquiries to establish the full circumstances and are keen to hear from anyone who witnessed any part of the incident or saw those involved in the area.
“This has happened alongside a busy main road in the early evening and there will have been a good deal of passing traffic.
"We would appeal to anyone who was travelling past the scene, particularly anyone with relevant dashcam footage, to contact us.
“While the investigation remains at an early stage, there is nothing at this point to suggest a link to any other previous similar incidents.
“Clearly serious violence involving weapons is completely unacceptable and we are working closely with our colleagues on the local neighbourhood policing team who are increasing their patrols of the area to reassure residents and continuing to support proactive operations to target those carrying weapons on the streets.”
Anyone with information is asked to contact Leeds District CID via 101 quoting crime reference 13210220241 or online via www.westyorkshire.police.uk/101livechat
Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.
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