Police and ambulance were called into Whitcombe Street around 8:25 p.m. Saturday night.
Three men, all 23, were taken to hospital. One of the injured is said to have fatal injuries.
A 19-year-old man has been arrested for attempted murder. Another 19-year-old man was arrested on suspicion of a brawl and grievous bodily harm.
Detective Inspector Emma Hampton said: “Our officers will be working beyond tonight and into the early hours of the morning trying to work out more details on this incident, but we have already made two arrests.
“We will be conducting full investigations and I urge members of the public to contact us to provide their accounts of what they saw tonight.
“If you have video footage of the incident, we would like you to contact us. “
In a tweet on Saturday night, Rhondda Cynon Taf’s head of council Andrew Morgan told people to avoid the area.
He said: “Have been made aware of an ongoing incident in #Aberdare near the city center where there is a strong presence of emergency services. Please avoid the area if you can for the next few hours. “
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