A 26-year-old man was stabbed to death as he travelled to hospital with his young daughter to visit his wife who had just given birth to a boy.
He became involved in an argument with a second man and died "in front of his daughter's eyes" in Croydon, south London, police said.
A 22-year-old man was arrested and is in custody at a south London police station.
Det Ch Insp Cliff Lyons described the murder as "particularly tragic".
He said the man's three-year-old daughter had not been injured in the attack on Thursday evening.
The man was attacked outside a Lidl supermarket in London Road, close to West Croydon railway station, at 1815 GMT.
The victim was taken to Mayday Hospital where he was pronounced dead an hour later.
Police appeal for witnesses
Mr Lyons said: "This was a shocking attack in a public place in the early evening.
"The victim's three year-old daughter was with him when he was attacked, though fortunately she wasn't physically injured."
He added: "Even more tragically, his partner had given birth to a son just that morning. The investigation is at a very early stage and I'm keeping an open mind about the circumstances of the attack at this point.
"A number of witnesses have already come forward but I believe there are still people with information we need to speak to and I urge those people to get in touch."
Mr Lyons added that police wanted to talk to two teenage boys who were at the bus stop when the incident happened.
He said: "They are described as black, aged between 15 and 20 years old, of slim build and wearing hooded tops."
Mr Lyons said officers were still trying to find out if the man had seen his newborn son before he died.
He said London Road, where the incident occurred, was "one of the busiest roads in the area".
Officers have found a number of knives in the area and were also reviewing CCTV footage from a number of cameras.
A post-mortem examination was being carried out on the man.
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