Police have pulled out of Belfast's New Lodge area after contractors unsuccessfully attempted to remove an anti-internment bonfire.
Youths threw missiles at police Land Rovers as they left the estate at speed at around 3pm on Thursday.
The estate was later barricaded using metal fencing while teenagers held their hands up in triumph from atop the anti-internment pyre.
Police had moved in earlier to facilitate contractors who have been tasked with removing the structure.
At one stage an officer collapsed after he was hit by an object at close range during sporadic violence.
The Chairman of the Police Federation for Northern Ireland, Mark Lindsay said officers were trying to protect the community and condemned the scenes witnessed.
"They are being attacked with a range of missiles and iron fencing," he said.
"In one assault by a mob, one officer is seen to be struck and knocked to the ground.
"This behaviour is contrary to the overwhelming wishes of the people of New Lodge who do not want this bonfire or their area hijacked by young thugs."
The bonfire was set to be lit on Thursday evening to mark the anniversary of the introduction of internment in 1971.
"Our officers are acting with professionalism and restraint and we would urge those orchestrating these vicious and unacceptable confrontations to pull back before people are seriously injured," said Mr Lindsay.
"These are ugly scenes reminiscent of the past and do not need to be repeated as communities work to build a better future. They are disgraceful and unacceptable. I know local community and political figures are trying to intervene to get order restored and from our perspective, we hope they are successful.
"In the meantime, our officers will continue to do the job they are there to do.”
The PSNI said it was there to support the bonfire removal contractors who were there at the request of landowner, the Department for Infrastructure.
A spokesman said the aim was to allow them to complete the task "with minimum disruption to the community."
The bonfire was built on land owned by the Department for Infrastructure.
Politicians from Sinn Fein and the SDLP spoke earlier this week and said people in the area are against the bonfire.
Graffiti appeared close to the pyre on Tuesday, which threatened that if the structure is removed then a nearby community centre will be targeted.
The graffiti is set to be removed by council workers.
A spokesman for Belfast City Council said it would remove the offending graffiti "as soon as practicably possible".
Operation Demetrius in 1971 saw hundreds arrested across the region on suspicion of being involved with paramilitary groups.
The vast majority arrested were nationalists, although a significant number of them had no connection with the IRA.
The anniversary has traditionally been marked by many from the nationalist and republican tradition with bonfires, although recent years have seen a move away from pyres towards community-based diversionary activities.