It is near the end of what Protestants call the summer "marching season" in Northern Ireland, and they took to the streets here today for what is a yearly commemoration of one of their ancient victories over Irish Catholics, this one in 1689.

Catholics responded this afternoon in a heavy skirmish with police and British troops who came under a hail of molotov cocktails and retreated under cover of their own barrage of rubber bullets.

Until 20 years ago, many local Catholics -- more amused by the spectacle of the parade than angered by the reason for it -- used to trudge up the steep streets of this walled city to view the dozens of marching Protestant bands.

That more peaceful time now seems longer ago than the 1689 battle itself, however. Today's parade, winding through the now-segregated city on a path dangerously close to Catholic neighborhoods, was carried out under heavy police guard. At intersections where it came within Catholic view, police spanned the cross-streets with black curtains.

But the curtains could not drown out the sound of the drums, beating an hours-long militant tattoo punctuated by angry anti-Catholic cheers.

The result was as predictable as the cold drizzle that fell. As the marchers dispersed and headed east across the River Foyle bridge to the Protestant side of town, the Catholics reestablished their claim to the western bank.

The fighting began as it has so often since "the Troubles" started in Northern Ireland 16 years ago. This week, it has had a special intensity on both sides following the death here Tuesday of an Irish Republican Army activist whose bomb blew up too soon, and his funeral two days ago, where a leading American IRA backer banned from the province by the British slipped into the country and helped carry the coffin right under the British Army's nose.

It was the largely Protestant Royal Ulster Constabulary police force, however, who seemed to have provoked the violence with a show of force today. Shortly after the parade, a group of their armored Land Rovers drove slowly down a street where they rarely appear in the center of the main Catholic neighborhood, the Bogside.

As they passed a high-rise apartment building, scheduled for demolition and mostly uninhabited, they were hit by a gasoline bomb from above. The police retreated, only to be replaced quickly by many more armored vehicles of the British Army, each carrying a half dozen troops.

Under the hostile but relaxed gaze of a streetside audience of hundreds from the Catholic community, the Army vehicles assembled at the end of a wide boulevard. About 100 yards away stood the apartment building.

The soldiers slammed the armor-plated doors of the vehicles, revved the engines and tore down the road, coming to a screeching halt in front of the building.

More gasoline bombs rained down, spreading flaming fuel across and around the vehicles. The crowd giggled and cheered.

The soldiers jumped out, one of them stepping into the flames and catching his feet on fire. As he hopped across the road, the crowd laughed.

British policy calls for use of usually nonlethal rubber bullets for such occasions. According to regulations, they are to be fired toward the ground, intended to ricochet into the knees of their targets. The soldiers aimed straight up into the building and shot.

More gasoline bombs, pieces of furniture and window glass rained down on them. The soldiers jumped back inside their cars and headed to a vacant lot on the corner.

Young men in the crowd seized the car of television crew nearby, dragging it into the road below the windows. Those above began pelting it.

More constabulary vans arrived and were hit by more molotov cocktails and debris. Suddenly, a bathtub flew out of a window and crashed below. The crowd roared with laughter, and somebody remembered that during a similar scene earlier in the week a stove had been thrown.

After more retreats and feints, the police inexplicably left. The crowd decided that the show was over and slowly dispersed.