With some justice, Britons believe they inhabit a tolerant, humane and civilized society. But two events this week - Amnesty International's charge of police brutality in Ulster and Sunday's assault by a white mob on Asian immigrants in London's East End - point to a darker side that Britons would prefer not to know.

Prime Minister James Callaghan's government, ever sensitive to the voters' mood, has taken pains to play down both occurrences and carefully refrained from sounding any alarms.

Amnesty's report, published yesterday but leaked to the press last week is a well-documented account of what appears to be the systematic use of force by police in Northern Ireland to extract confessions from terrorist suspects. The government from terrorist suspects. The government has rejected Amnesty's proposal for a public inquiry but will appoint its own man to look into the carges behind closed doors.

Since Roy Mason, the minister in charge of Ulster, frequently boasts of the success of his police, there is considerable skepticism about the outcome of this investigation.

In privat, Mason would probably say, "Results count and I get them." It is true that nearly every person now charged with a terrorist offense in Northern Ireland is found guilty by the special one-judge, no-jury courts there. In most cases, moreover, the convictions rest largely on confessions.

The outcome, Mason would say, is a sharply reduced level of violence by the Provisional Irish Republican Army. Two years ago, there were four killings every five days: now there is one. The bombing rate of two a day has been cut in half. Violence in Ulster has all but disappeared from the front pages in the rest of Britain.

Indeed, Mason could find no stronger testimonial than that of his onetime arch-enemy, Seamus Twomey, the ex-bookie who ran the provisional Irish Republican Army until police in the Irish Republic caught up with him.

Early in the week, Twomey was on trail in Dublin again and the police read out "staff report" he had written to reorganize the IRA. It said that Ulster police were "breaking volunteers" with their interrogation, and the IRA must somehow imbue them with more "psychological strengthe."

Thoughtful observers in Britain and Ulster wonder, however, whether Mason is making short-term gains and incurring long-term losses. There are fears that the reputed methods of his police are seriously alienating Ulster's half million Catholics and even some Protestants, a fraction of whom also fall into police hands.

The tough tactics have no doubt crippled the IRA's "trained" soldiers. But it is unknown how many new ones are recruited as word of the alleged abuses spreads.

The tactics raise questions for the rest of Britain. If allegedly brutal police methods are thought to pay off in Ulster, won't police elsewhere be tempted to copy the techniques?

Callaghan, however, is convinced that Mason has made the best of a tough job. He has pleased most of the 10 Ulster Protestant members of Parliament and many frequently vote with the government. This is no small blessing to a minority government.

Indeed, Labor's lack of a majority also accounts in part for the tepid response to Sunday's racial outbreak. The white youths who stormed Brick Lane with clubs and bottles, crying, "kill the black bastards," mostly come from the public high-rise complexes that vote overwhelmingly Labor.

There is little doubt that Callaghan and virtually every other Labor member of parliament detest racism as a moral outrage. But there is equally little doubt that Labor politicians fear they will lose some of their traditional supporters over the race issue.

The Tory leader, Margaret Thatcher, is making a powerful bid for the alienated public housing voter by her stern denunciation of nonwhite immigration.

So after the Sunday rampage, the government declined even to make a public statement. Callaghan's home secretary and political confidant, Merlyn Rees, quietly called for a report from the police.

Minority government has been widely praised here as a guarantee against extremism. If Labor controls less than half the House of Commons, the argument runs, its ideological left must behave and give a free hand to centrist like Callaghan. Similarly, it is said, a minority Tory government will squelch its ideological right and hew to roughly the same center line.

But it is now evident that there are costs as well benefits from minority rule. One might well be the premium it places on quick and easy answers and the weakening of values that Britons have taken for granted.