Ten years ago Monday, a determined but relatively unknown member of Parliament, Margaret Hilda Thatcher, challenged the established leaders of Britain's then-opposition Conservative Party and won the right to lead it.

Today, after almost six years as prime minister, Thatcher and "Thatcherism" dominate and divide British politics and public life.

The deeply conservative 59-year-old grocer's daughter who still "lives above the store" at 10 Downing St. is, by all accounts, a formidable political leader: tough-minded, articulate, well-informed and relentless in pursuit of her goals.

She may well be on her way to becoming Britain's longest continuously serving prime minister, having three years left of her second term and apparently in good shape to win a third five-year term in the next election, which she says she wants to do.

She is blessed with a fractured, and some say politically suicidal, opposition. This enables her to hold a huge 141-seat majority in the House of Commons, even though only about 43 percent of voters backed her in the 1983 election and about the same percentage now tell pollsters they support her.

Yet, to many people, including some of her admirers, Thatcher has a serious political flaw -- usually described as an abrasiveness that manifests itself in a desire for total victory, a guilt-by-association style of rhetoric aimed at her enemies, and a seeming lack of compassion for those outside her cult of individual initiative.

It is all the more curious because Thatcher's extremely secure political position would appear to make such a stern approach unnecessary.

Many commentators believe it contributes to and may even accelerate the growing polarization in Britain between the decaying industrial north and the prosperous south, and between the record 13.9 percent unemployed and the rest of the work force.

Indeed, if a line were drawn across Britain from south London in the east to south Bristol in the west, one would find no Labor Party parliamentary representatives to the south and no Conservative control of any major city to the north.

Her aides and others close to her said it is just Thatcher's style, and does not mean that she does not care. One argued that the British people understand Thatcher's personal toughness and believe it also will toughen up the country.

Nevertheless, 10 years after her success in gaining control of her party and almost six years after taking the national helm, four aspects of her stewardship seem to dominate:

* She has become the most radical prime minister in many years and is beginning to leave marks on British society that probably will endure. These include reducing the degree to which people look to government to solve their problems, rolling back controls ranging from those on overseas investment to wages and prices, tackling head-on the power of big labor unions and selling off about $8 billion worth of state-owned industries.

She is not "a broad-gauged or inspirational leader," said Prof. David Butler of Oxford University, "but the complex of British life has changed, and she has unquestionably left more of a mark than most."

* Her personality alienates many people, from average Britons to a number of foreign leaders with whom she has tangled in Ireland and Western Europe, and threatens to obscure or overturn what she is trying to accomplish for Britain at home and abroad.

* Her total dominance of the political scene here, and the relative weakness of the opposition, have raised serious concern that there is not only no coherent alternative to Thatcherism being voiced, but also that significant minority groups and opinions have few channels through which to be heard.

Debate in Parliament is now dominated by shouting matches richer in rhetoric than substance, in the view of many observers. "Going into 1985," wrote Manchester Guardian columnist Hugo Young, "Britain is consumed by the politics of rancor but deprived of the politics of argument."

* It is still not clear whether Thatcher will succeed in shaking up her compatriots -- in making British industry more competitive and reintroducing what she calls an "enterprise and entrepreneurial culture" like that of the United States into a country that has had a substantial dose of socialism in recent decades. The prime minister has succeeded in slashing inflation from 20 percent to 5 percent, although partly at the cost of 3.3 million unemployed. British growth has continued, slowly, despite the recession of a few years ago.

But it is not certain that Britain as a whole is cut out to respond to Thatcher's brand of aggressive global marketing and competitiveness, and her aides acknowledge that it may take longer than the lives of two Parliaments -- 10 years -- to find out.

Thus, despite her unquestionable political power, Thatcher is a figure of ambivalence -- admired by many for her grit and goals, despised by many who see her as cruelly disregarding the plight of the have-nots, and frustrated by many who see her as unnecessarily confrontational.

This ambivalence is perhaps best described by one of Thatcher's major targets, Ken Livingston, the socialist leader of the Greater London Council whose job the prime minister is eliminating.

"I think I admire the determination," Livingston told a British Independent Television interviewer last week, "because all the track record of the last 30 years is a series of prime ministers who are just blown backward and forward by events. Now Mrs. Thatcher's tried to impose her personality on events and the Civil Service, and that I find very admirable. It means that people know what they're getting when they vote for her."

Thatcher, Livingston continued, is among the small group of leaders "who are able to articulate hopes and fears, and they change hearts and minds and shift political opinion. I think Mrs. Thatcher detected, after 30 years basically of the welfare state, a growing disillusion with bureaucracy, a feeling that people had to pay too much in tax, and she very skillfully turned that against socialism, though it wasn't all the fault of socialism by any means. But people can identify with it. . . ."

But, he added, "If you've got that degree of determination and ruthlessness, you have to be much more sensitive to the moods of public opinion. You've got to be more than the average prime minister and be acutely aware of the broad consensus that exists in the country" and that is what Thatcher does not have, he said.

From the start, Thatcher has been an outsider. Although an Oxford graduate, she was a chemist and not a member of the landed gentry, or upper class or traditional governing elite. Thus, her success and her appeal has eroded not only the solidarity of the labor unions, about a third of whose members voted for her, but also the establishment, some of whom would prefer a more moderate and traditional brand of Toryism.

"Her government has brought the country face to face with its decline and administered a shock to the system, which has been most salutary and overdue," wrote Peter Jenkins, another respected Guardian columnist.

"But never has she found the words to clothe her policies in national purpose in which lies the art of Tory government."

Her closest associates shrugged that off.

In 1979, when she came to office, one top aide said, Thatcher "felt this was the last time it would be possible for a Conservative government to halt the tide of socialism that was turning Britain into a modern serfdom in which ev-eryone was beholden to the state. It's not her scene to be lovey-dovey and soft. She wasn't brought up that way, to be spuriously compassionate, and it sounds breathy and false when she tries it.

"It doesn't mean she doesn't care," he added. "But she has a different way of caring and that is to put the country in a position to earn its keep."

Thatcher's conservative ideology, her dedication to capitalism, individual initiative and to remaining a politician who leads by "conviction" rather than always seeking consensus, frequently have led to comparisons with President Reagan.