A police constable was shot dead in cold blood in Khayelitsha on Tuesday morning after walking a mentally ill person home, Western Cape police said.

He is the fourth police officer to be shot dead in the past ten days.

"The constable and his partner, also a constable, had just returned to the police van after assisting a person, allegedly mentally ill, to his home in the same street," said Senior Superintendent Billy Jones.

"They were approached by an unknown armed man who immediately opened fire on them."

The incident happened in Phambile Street in Lingelethu West at Khayelitsha around 3am.

"[The] constable was fatally shot in his head while sitting behind the wheel of the police van," said Jones.

The man stole his service pistol before running off.

The constable's partner was not hurt.

"We have launched a search for the suspect. Anyone with information can contact Crime Stop on 08600 10111," said Jones.

Last week, a policeman was shot dead while on duty in Durban's Umlazi suburb. The 52-year-old detective, Inspector Simphiwe Manyoni, had been assisting a complainant on Sunday when he was shot in the head.

Another top cop, Dries van Tonder, who had secured 100 convictions, was shot dead in Bez Valley, Johannesburg last week after arresting a man with an AK-47.

At the weekend, a 27-year-old student constable, Valerie Zitha, who was on her way to an anti-crime prayer meeting, was shot on a train between Mamelodi and Silverton and later died in hospital.