There is a heavy police presence in Canton, Cardiff, with reports of armed police and a helicopter in use.
The junction of Atlas Road was cordoned off and Lansdowne Road closed. The cordon has since been lifted but there is still a police presence.
The nature of the incident is unclear at this time.
Armed police have arrested four men on suspicion of possession of a firearm at Talbot Green.
South Wales Police responded to an incident at a car wash in Swansea at around 2:40pm on Tuesday after receiving reports of a group of men in a 4X4 had made threats with weapons.
Officers later pulled over a vehicle which matched the description at Talbot Green and the arrests were made.
South Wales Police have stressed it is not being treated as a terrorist incident.
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Two men have been arrested following a 'serious assault' in Grangetown, Cardiff in the early hours of Wednesday 17th May.
South Wales Police were called to Allerton Street where they arrested the 46 and 52 year old men.
One man was taken to hospital following the incident. He is not thought to have sustained serious injuries.
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The victim of a fatal road traffic collision on the new Aberfan by-pass on Saturday evening has been named as 23 year-old Andrew Lewis from Aberfan.
Andrew was a much loved son, brother, father and uncle.
He was loved by everyone that knew him and the family are in total shock and totally heartbroken following the tragic death of Andrew.
A red BMW F650 motor cycle, on which Mr Lewis was a passenger, and a red Ford C-max motor car were in collision near the local authority amenity centre. The 26 year old male driver of the motorcycle is at the University Hospital of Wales in Cardiff being treated for serious injuries.
The driver of the Ford C-Max was uninjured.
Police are appealing for witnesses.
A motorbike passenger has died after a collision at Golwg-y-Afon, Aberfan on Saturday evening.
A 23-year-old died when the red BMW F650 motorcycle collided with a red Ford C-max car near Ynysowen Community Primary School and the Local Authority refuse and recycling tip.
The other person on the motorcycle, a 24-year-old male, is currently being treated in hospital for serious injuries.
The female driver of the other vehicle was uninjured.
Andrew’s family has paid tribute to him, saying: “Andrew was a much loved son, brother, father and uncle.
“He was loved by everyone that knew him and the family are in total shock and totally heartbroken following the tragic death of Andrew”.