IN ROME LAST WEEK, students fought with police and a sniper killed a policeman. In London over the weekend an organization called the National Front held a rally demanding the expulsions of blacks and Asians from Britain. Italy and Britain remain the most threatened of the major Western European economies - those with the highest inflation rates, the largest trade deficits and, at least in the case of Britain, the widest unemployment. There is more to international economics than numbers on a chart. These countries are now under great and growing strain, and one indication of it is the resort here and there to perverse politics. The real explanations of Europe's slack times are less appealing, in the streets, than the rhetoric of the irrational is.

Neither of these incidents constitutes a trend. Neither amounts to anything more than a flickering of the yellow warning light. The racist rally in London drew perhaps a thousand demonstrators. They were considerably outnumbered by the police, who more or less successfully prevented counter-demonstrators of the left from getting at them. The street fighting in Rome was, in the Italian manner, more dramatic but also more ambiguous. As with the other violent protest movements in Western Europe, the Roman student radicals are altogether isolated from any wider support. At this point they are largely victims of themselves. Over the past decade the students have very nearly destroyed the University of Rome with their demands for open admission and debased grading. As a result, not altogether unnaturally, the university's graduates are having even more trouble finding jobs than other young Italians.

There have been other outbursts of radical violence in Italy in recent years and, in the past, they have faded away. But the current trouble is having a greater impact because of the extremely fragile character of the present conservative government. It stays in power only at the sufferance of the Communist, and the radicals' taunts and abuse are beginning to get under the Communists' skin. Until late think that it had plenty of time and could continue in the Communists are apparently beginning to wonder suffering serious losses of prestige and authority among their own following.

Three months ago the United State was talking briskly about a concerted worldwide drive to spped up economic recovery and to lift production. But now the Carter administration has dropped most of its own plans for cranking up business activity here this year. The dominant nations, the United States and West Germany, agree that they cannot risk further inflation. As a result, the summit meeting of the rich nations in London next month is unlikely to contribute much more than carefully phrased generalities to the condition of the weaker countries of Western Europe. The outlook there is, as the Weather Bureau might put it, a 75 per cent chance of more ugly street demonstrations as long as unemployment is high and standards of living are declining.