Police Commissioner Kenneth Newman put Londoners "on notice" today that he will authorize whatever methods necessary, including the use of plastic bullets and tear gas, to prevent a recurrence of the rioting that shook the north London neighborhood of Tottenham last night.

Newman's warning typified a get-tough line taken by officials in the aftermath of the fourth -- and by far the most violent -- such outbreak in Britain in the past month.

While acknowledging the economic deprivation of the predominantly black inner-city areas where the disturbances have taken place, and the possibility that incidents involving police tactics may have sparked them, a senior government official said there was "no shadow or trace of an excuse for the crimes which followed."

As Tottenham residents ventured outside in a cold drizzle today to begin clearing debris, however, community leaders blamed government policy and law enforcement methods.

They charged that massive unemployment along with racially motivated police harassment have driven local youths to explosive despair.

Last night's riot was described by officials as the most serious outbreak on the British mainland. Television footage of flaming buildings and automobiles, shielded and helmeted police, shotgun blasts and hurled firebombs shocked Britons who are used to such scenes only on the streets of Northern Ireland.

The level of violence went far beyond the urban rioting that swept the country in 1981. Last night, a policeman was killed, "hacked to death" by machete-wielding rioters, in the words of an eyewitness. Seven people were wounded by shotgun fire, the first time firearms have been used by rioters on mainland streets.

The scale of battle in Tottenham, the latest in a series of violent disturbances in economically deprived inner-city areas in the last month, was indicated by morning-after statistics. More than 230 people, mostly police, were injured, many seriously. Only seven arrests were made. By contrast, a riot in the south London district of Brixton a week ago resulted in 50 injuries and 250 arrests.

In the sometimes emotional assessments offered from all sides today, police officials charged that "Trotskyites and anarchists" from outside the high-rise public housing complex where the violence was centered helped organize the riot. Cases found in the rubble, apparently used to store the components of gasoline bombs, showed the disturbance had been planned, some officials said.

Deputy Assistant Police Commissioner Richard Wells said charges that the police had helped provoke the riot were "a figment of some very strange imaginations."

In apparent response to criticism by spokesmen from the opposition Labor Party, Wells said that "one senses here ideological hands being rubbed in glee" over problems for Prime Minister Margaret Thatcher's Conservative government.

In a news conference this morning, London Metropolitan Police Commissioner Newman confirmed reports that "members of the tactical firearms unit" had been deployed in Tottenham last night, although they were not used.

The unit is equipped with plastic bullets that are designed to stun and incapacitate but not kill their targets. The bullets have never been employed in England, Wales or Scotland, although they are in regular use by the British Army and police in Northern Ireland, where they have been held responsible for at least 12 deaths.

Newman said police surrounded and eventually occupied the high-rise complex without having to use the weapons. He and other officials praised the police involved for their restraint during several hours of "ferocious" rioting during which they largely adopted a stationary containment policy while a barrage of missiles, including bricks, concrete slabs and cans of food, in addition to the firebombs, rained down on them.

But in the future, he said, "I will not shrink from such a decision should I believe it a practical option for restoring peace and preventing crime and injury." Yesterday's events, he said, are "unacceptable, as far as I'm concerned, and will be dealt with with the utmost firmness."

Home Secretary Douglas Hurd called Newman's decision a "sad step," but said he supported it to protect police officers seeking to quell riots. In addition, he said, peppery CS gas -- deployed only once before, during the 1981 disturbances -- also would be used.

Hurd said incidents between police and local residents that had preceded the Brixton and Tottenham riots were being investigated. But such incidents, he said, "provide no shadow or trace of an excuse for the crimes which followed."

The Sept. 28 riot in Brixton was sparked when armed police forcibly entered the home of Cherry Groce, a black woman, in search of her 19-year-old son, Michael. Told that Michael might be armed, a nervous officer apparently shot Mrs. Groce when she appeared in a doorway. Michael was not at home. Mrs. Groce remains in the hospital, paralyzed from the waist down.

In a raid Saturday, unarmed police entered the Tottenham home of Cynthia Jarrett, a 49-year-old West Indian, in an unsuccessful search for stolen property in connection with her son, who was under arrest. Jarrett, heavy-set and with high blood pressure, collapsed during the raid and died, after an apparent heart attack, en route to the hospital.

Police said they tried to resuscitate her but would have no further comment pending an investigation. Family members present said Mrs. Jarrett had been shoved to the floor by police. According to Bernie Grant, the elected leader of the Haringey Borough Council, where Tottenham is located, the four policemen "actually stood by as she lay dying on the floor for some 15 or 20 minutes."

Grant charged today at a news conference that police initially had refused to call an ambulance on their radio, and "stepped over her body to go on searching the house."

Community activist Dolly Kiffin said the raid "didn't have to be violence. It didn't have to be a death. It didn't have to be that way, if the police just use their common sense."

Although many here say factors other than race play a role in disturbances such as those in Brixton and Tottenham, and a number of white youths also participated in the Tottenham violence, relations between white Britons and immigrants from former colonies in the Caribbean, India, Pakistan and Africa have never been smooth. The vast majority of Britain's 3 percent nonwhite population arrived here during the 1950s and 1960s, between the period of postcolonial independence and the introduction of immigration laws that have become more restrictive.

Most settled in urban areas, nearly half of them in London, where the nonwhite population is nearly 15 percent. While many Pakistanis and Indians moved to the outskirts of the capital, where they have suffered their own racial problems with working-class whites, most of the Caribbean blacks have congregated in public housing in a few inner-city neighborhoods like Brixton and Tottenham.

It is the British-born sons of those immigrants, as many as two-thirds of them unemployed, who have played a large part in both the 1981 and the most recent riots. They repeatedly have come into conflict with urban police forces primarily composed of working-class whites. In metropolitan London, where only 271 of nearly 27,000 police officers are black or Asian, helmeted police have been known to refer to riot duty as "coon-bashing" and to charge rampaging youths with chants of "nigger."

Following the 1981 riots, a government-appointed inquiry recommended a number of steps, including increased minority representation on urban police forces, the establishment of police-community liaison groups and more government money spent on urban programs and jobs for youths.

Since then, police recruitment of blacks has been largely nonexistent, partially because of the reluctance of blacks. Although the liaison groups were set up, "they are primarily talking shops, a PR effort," Ansel Wong, the principal race relations adviser to the Greater London Council, said in an interview last week.

The central government has put substantial funds into the urban areas for jobs and renewal. But most major cities and boroughs, all controlled by Labor Party councils, have experienced a net reduction in resources under the budget-cutting Thatcher government.

Although some race-equality laws do exist, there are few means to counter widely alleged racial discrimination in employment and no affirmative action regulations in Britain.

"It's a multifaceted problem with historical roots," Wong said.