A police raid in central London in which an innocent man was shot and critically wounded has left the British press and public deeply shocked and wondering what has happened to the country's general policy of not arming policemen.

In Parliament today, Home Secretary William Whitelaw called the episode, which occurred during the Friday night rush hour, "a most serious, grave and disturbing incident--nothing like it must happen again."

What happened Friday was a case of mistaken identity. As early evening traffic inched through the streets of London's elegant Kensington residential district, a police raiding party, guns blazing, attacked a small yellow car. One of the three occupants of the vehicle described the police as "exultant, really hysterical about the shooting."

When the firing stopped, a 26-year-old film editor, Stephen Waldorf, lay sprawled on the pavement, critically wounded.

Only then did the police gradually realize that there had been what Scotland Yard later called a "tragic case of mistaken identity." Waldorf was an innocent victim, thought by police to be an escaped suspect in an attempted murder case, David Martin, because a girlfriend of Martin's was in the car.

"You've made a terrible mistake," the woman, Sue Stephens, who was grazed by a bullet herself, screamed at the police, " . . . turn him over and look at his face." When they did, the officers went ashen. "At first they had looked really hyped up," Stephens said, "nervous but satisfied they'd done a good job. Then it was just horror and fright."

Waldorf today is fighting for his life with wounds in his liver, skull and chest. Three police detectives have been suspended while a major investigation is underway into what was plainly a disastrous police blunder. Martin is still at large.

And Britain is plunged into a searching examination of what has happened to its policy of unarmed police forces, one of the country's proudest traditions. "Bobbys," in their distinctive blue helmets, most carrying only nightsticks and walkie-talkies, are considered by their countrymen to be symbols of a civilized society, lacking the "Wild West" violence of their American cousins.

Reflecting the widespread outrage here, the liberal Guardian newspaper said in an editorial today, "Not only may he a policeman be armed, but he apparently feels he can open fire without provocation in the middle of a busy London street."

In fact, the case shows that while an increasing number of London--and British--policemen are trained in the handling of guns, the use of firearms by police here is still remarkably rare, certainly by U.S. standards. That may explain why this incident has caused such an uproar.

In all of London (with about 8 million people) in 1980, according to Scotland Yard figures, 28 shots were fired in six incidents with two persons hit. Six shots were fired in two incidents in 1981 and no one was injured. Through September 1982, four shots were fired in three incidents and one person was hit. That person was the now escaped David Martin.

Training police in firearms was begun in the early 1970s and today it is estimated that about one in every 10 British policemen--or around 12,000 officers--are authorized to use guns. The number for London is about twice that high, partly because of armed units for protection of the royal family, diplomats, antiterrorism and undercover work.

But two of the policemen suspended in the Waldorf shooting were regular district officers. Large stations keep only two or three weapons on hand in safes and they are issued only on approval of senior officers. On average, police figures show that guns in London are issued about 20 times a day, but are almost never drawn or fired.

Outside London, the highest reported issuance of guns to policemen is still less than once every three days. (None of these figures apply to Northern Ireland where British troops and police often use firearms against suspected terrorists.)

Moreover, there is little doubt that rigid police guidelines for firing were not followed in this instance. The rules require that warnings must be sounded by an officer first, and then "to reasonably protect himself or give protection, he may resort to firearms as a means of defense."

Critics say that the increased training of policemen in gun use ensures that more guns will be used both by criminals and the officers. "It is a cardinal principle of English law that the police will only use the minimum necessary force to effect an arrest," Conservative legislator John Wheeler said.

In the great majority of cases--and in spite of the Kensington tragedy--that still appears to be true.