Irish nationalist terrorists, expanding their latest campaign of violence here to purely civilian targets, today planted three bombs in the crowded Oxford Street shopping district, central London's busiest and best-known commercial thoroughfare. One of the bombs exploded, killing a police bomb disposal expert.

Kenneth Howarth, 49, died late this afternoon while trying to defuse a bomb that exploded in the basement lavatory of a Wimpy fast-food restaurant. The outlawed Provisional wing of the Irish Republican Army claimed responsibility for the attack after an earlier telephone warning that bombs had been planted in the restaurant and in two large department stores.

As the evening rush hour began, police evacuated thousands of shoppers, office workers, tourists and vacationing school children from more than a mile of Oxford Street, which usually is packed with people from morning to night. Unexploded bombs were found in the Debenhams and Bournes department stores identified in the warning telephoned to the Reuter news agency an hour before the restaurant explosion.

Today's attack was the third by the Provisional IRA in London during the past 16 days. The Irish nationalist paramilitary group has escalated its terrorist warfare against British rule of Northern Ireland following the end of the seven-month hunger strike for political prisoner status by convicted IRA terrorists in the Maze Prison outside Belfast. IRA guerrillas are engaged in a campaign to oust the British from Northern Ireland.

On Oct. 10, two passersby were killed and 39 people injured by a bomb that exploded outside the British Army's Chelsea Barracks, about two miles south of today's attack. The target of that blast was a military bus carrying 23 members of the Army's Irish Guards, 21 of whom were wounded.

On Oct. 17, the commander of the Royal Marines, Lt. Gen. Steuart Pringle, was seriously injured, losing his right leg below the knee, when a bomb exploded in his car as he drove away from his suburban home. He is now in satisfactory condition.

Today's attack was the first by the Provisional IRA on only civilian targets in England since December 1978, when 10 people were injured by bombings in five cities. The Provisional IRA frequently struck in English cities, particularly London's commercial West End, during the mid-1970s but later abandoned this tactic as counterproductive.

The scene in central London this evening resembled those in Belfast's business and shopping district during much more frequent bombing incidents there. While the search went on for other bombs after the first explosion, masses of police cordoned off the area. People pouring out of stores and buildings onto the sidewalks were urged by loudspeaker to "please leave Oxford Street as quickly as you can. There is the possibility of another explosion in the street. Keep away from police vehicles and glass storefronts."

A man who identified himself as a member of the IRA telephoned Reuter shortly before 3 p.m. and told switchboard operator Marianne Mampel, "There are three bombs in Oxford Street." She said he then slowly described in detail where each was supposed to be located.

The manager of the Wimpy hamburger restaurant said police arrived suddenly and evacuated 150 customers and 27 employes. A police spokesman said the bomb was found in two paper bags exactly where the caller to Reuter had said it would be.

Howarth, the disposal expert, arrived just after 3:30. Nigel Gammon, 20, who had been painting a sign across the street from the restaurant, said Howarth was "laughing and joking" with other police officers as he put on his flak jacket and protective helmet. "He then went down into the basement and we heard a big bang," said Gammon. "It was awful."

The explosion blew out the front of the restaurant, scattering glass and debris but not seriously injuring any of the hundreds of bystanders kept back by police lines. "There was a bang and then smoke," said Ruth Selber, who works in an office just off Oxford Street. "You could smell cinders. Everybody was screaming and running. Oxford Street was very crowded at the time and people were running in all directions."

The Provisional IRA's statement claiming responsibility for the attack, issued through the Irish Republican Publicity Bureau in Dublin, said its warnings should be better heeded in the future. It also referred to Irish Catholics who have been killed or injured by plastic bullets used by British soldiers and Ulster police in response to stone and firebomb attacks in Northern Ireland. The bullets are meant to be aimed below the waist and from a distance to avoid fatalities, but they have killed several adults and children in Belfast's Catholic neighborhoods in recent months.

"Let the British people take note that the Irish children, the victims of plastic bullets fired by their soldiers, do not have the luxury of receiving warnings," the Provisional IRA statement said. "In future, when we give warnings, respect them."

"The British people should press their government to withdraw from our country," the statement added. "Then there will be no bombs in London and there can be peace in Ireland."

The bombing was condemned as a "cowardly act" by Deputy Prime Minister William Whitelaw, who, as home secretary, is Britain's top law enforcement official. He said such attacks were "so easy and simple to carry out, and so very hard for all civilized countries who preserve freedom to prevent."