Although he was stabbed in the leg, the policeman drove away tonight with the robbers, who eventually fled empty-handed.
The 37-year-old, who was not named, was in a stable condition when he recovered at the hospital last night.
Scotland Yard said the London official had been stabbed to death on Saturday at 5:45 pm in Lewisham, south London.
A yard spokesman said, "He came across an incident in which a group of four men on mopeds attempted to steal another moped from a 21-year-old man at Maylons Road in Lewisham, armed with a large kitchen knife ,
"The unarmed officer intervened and challenged the suspects." During the altercation, the officer was stabbed in the leg.
But still he continued to deal with the four suspects and thwarted the theft of the moped until the suspects revealed the scene.
"The officer was taken to the hospital.
"The 21-year-old victim of the attempted theft was also attacked and suffered bruising."
Detectives chasing the gang investigate CCTV footage from the area. They appealed to witnesses or people with information about the gang to call the police.
Moped crimes have risen sharply in recent years, especially in London.
At the beginning of this year, official figures showed that 60 daily robberies on motorcycles took place in the capital.
Among the prominent victims is the comedian Michael McIntyre, who was robbed in June in Golders Green in northern London in front of his son, 10, his Rolex watch.
The threat was highlighted by the Daily Express Stop the Moped Madness campaign, which was supported by the Prime Minister.
Steve O & Connell, a conservative member of the London Assembly, has called on London Mayor Sadiq Khan to impose zero tolerance on moped crimes.