A machete-wielding man was arrested as tens of thousands of young fans gathered for a Justin Bieber concert nearby.
A foot-long blade was discovered as police tackled the man as fans gathered in the packed streets.
Two men were arrested as footage emerged of police wrestling one of the men to the floor just 500 yards from the Principality Stadium in Cardiff.
More than 40,000 young fans are expected to attend the Welsh-leg of the Desposito singer’s Purpose world tour.
One witness said: “I walked past as there was lots of screaming and shouting as the police were there.
“I saw a guy get arrested and the police officer hold the knife – it was massive.
“It’s terrifying to think something like this could happen with so many people in town.
“It was very scary after what happened at the Ariana Grande concert.”
A manager at the Viva Brazil bar and restaurant, who didn’t want to be named, said: “There was a big police presence outside but it was all over very quickly.
“Thankfully no one was hurt. I wouldn’t like to say anymore because it’s obviously an ongoing police matter.”
A spokeswoman for South Wales Police said two men were arrested following a “motoring dispute”.
She said: “South Wales Police were called to a dispute involving motorists on Saunders Road, Cardiff, at approximately 3.05pm. Two men, aged 19 and 27, were arrested nearby on Penarth Road.
The 27-year-old has been arrested on suspicion of affray and the 19-year-old has been arrested on suspicion of using threatening behaviour. Both are currently in custody at Cardiff Bay Police Station.
“There are no reports of any injuries.”