After being pushed off his bike by one member of the public, Tarafdar was chased on foot by an officer and another passer-by.
A third man tripped him up and Tarafdar was brought to the ground and arrested.
He was later found to have a large hunting knife in his rucksack.
Andy Brown, who filmed the incident while he was waiting at traffic lights in a car,
told BBC Radio Wales: "It was bit surreal. We were just sat at the traffic lights, saw a police van and a police car racing round the corner.
"Next thing he was taken down by a member of the public, right in front of me.
"You could almost see a couple of people lining up to take him off his bike, the second guy saw him coming and put a foot out to trip him up."