Watch ITV Wales journalist Ellie Pitt report from Treorchy
Emergency services were called to what was described as a "serious incident" in the village of Ynyswen, Treorchy in south Wales at midday on Friday.
Police confirmed a few hours later that "a number of casualties" were involved with neighbours describing seeing around 30 emergency vehicles at the scene.
The incident occurred in Baglan Street at around midday on Friday and forensic officers were seen inside a Chinese takeaway restaurant in the afternoon.
On Friday evening, South Wales Police confirmed a 16-year-old girl had died in what they were describing as "sudden and unexplained". Two men have been arrested and were being treated in hospital for serious injuries, the force added.
Local builder and resident on Baglan Street John Belgrove told ITV Wales that he was working at home when he heard "a load of police sirens".
"My wife came home from shopping and said 'Have you seen the amount of police cars out the front?' We counted 31 emergency vehicles and we saw the air ambulance on Baglan field.
"For somebody who lives just a few doors away, it's quite alarming. I grabbed some cones and helped assist police cordon off the road. It was absolute chaos - vehicle after vehicle come in", he added.
"I've got children and it's a bit of a shock - I suppose it's a bit difficult to put into words."
People have been urged to avoid the area in Rhondda Cynon Taf and traffic was diverted away from the scene.
The Welsh Ambulance Service said it was called at midday to an incident and attended the scene with one rapid response vehicle, three ambulance, a hazardous response team and the Wales Air Ambulance.
Local resident Linsey Simmonds said, "I was coming past the Baglan Field and saw the Air Ambulance land there, and in the past 20 minutes there's been up to 30 police and ambulance vehicles."You can't walk past there or drive up the valley."
Air ambulance captured leaving the scene
The First Minister tweeted about the incident and called the incident "deeply concerning."
Mark Drakeford said he is being kept informed of any developments and his "thoughts are with everyone in the community during this worrying time."
He reiterated police advice for people to avoid the area.
Rhondda MP, Chris Bryant, said he had been informed of a major incident which he described as "disturbing events" and thanked the emergency services in attendance.
Mr Bryant added that emergency services are expected to be at the scene for "a very long time" and he hopes "nobody is extremely hurt".
The MP reassured that people in the area should not be concerned for their safety.
He said: "My concern is to make sure everybody in the Rhondda is safe, and the police have guaranteed that".
Leader of Rhondda Cynon Taf Council Andrew Morgan said his "thoughts are with everyone affected" and advised people to avoid the area until emergency services have left.
Anybody who may have witnessed the incident is being urged to contact 101 immediately.