A Kenyan politician has been shot dead by gunmen, along with two body guards and his driver, officials said.

George Muchai, the parliamentary member for Kabete, was killed in Nairobi's Central Business District in the early hours of Saturday morning as his family, who were travelling in a separate car, watched the attack.

Police are currently investigating the motive for the killing.

Kenyan President Uhuru Kenyatta mourned the loss of the lawmaker, who he termed a "diligent gentleman" and a true servant of the people.

"I expect the police to mobilise all resources to ensure the criminals who murdered Honourable Muchai, his two bodyguards and driver are speedily apprehended and made to pay," the president said in a statement.

Detectives said Muchai, a member of Kenya's governing Jubilee coalition, was returning home after meeting family members in a restaurant when the attack took place.

"The MP was coming from a family meeting in Westlands and was headed home," a police officer said, adding the MP's car had slowed down at a roundabout when another car rammed it from the side.

"Masked gunmen emerged and opened fire at them, killing all of them at once. They also stole a briefcase and two pistols from the bodyguards," the officer told the AFP news agency.

Central Police chief Paul Wanjama said the car carrying the attackers sped off after the shooting.

"Our medics who responded to the early morning cold-blooded murder found no signs of life in all the four," said Fred Majiwa, a spokesman for St John Ambulance.

A senior police officer said the killing may have been an execution-style hit.

"It is something we can't rule out at all. It seems to us like something that was well planned," said the officer, who asked not to be named.

Source: Al Jazeera and agencies