LONDON — It is, in its way, one of Britain’s great anachronisms: a university boat race steeped in pageantry and tradition with nothing but honor at stake, a moment when old buffers in satin-trimmed blazers can revel in upper-crust camaraderie and pretend that the important things in the world remain much as they were when the race was first run in 1829.

But the annual Oxford-Cambridge race along a historic stretch of the Thames in London has become more than a highlight of the sporting calendar. In recent years it has been the occasion for another contest, between those who view it as a grand spectacle and those who condemn it as an emblem of a class- and tradition-bound Britain whose day has passed.

Until this year, both contests were generally fought within what the British call the Marquess of Queensberry rules of fair play. To be sure, the boat race has seen sinkings, annulments, at least one dead heat and two crew mutinies, including one 25 years ago by American oarsmen that provided the plotline for the 1996 movie “True Blue.” And every year in recent times, noisy protesters have thrust their banners at the television cameras that broadcast the race and the festive scene on the riverbank to viewers around the world. But in the end, things had always come off peaceably enough.

Then came Saturday, when the whole affair ended more in frustration and tears than in celebration.

A bit more than halfway down the four-mile course, a bearded Australian in a wetsuit who had posted a 2,000-word essay on the Internet beforehand titled “Elitism Leads to Tyranny” jumped into the river and swam directly into the path of the two boats, which were racing neck and neck. The churning oars of the Oxford boat narrowly missed hitting him, but his presence caused sufficient alarm that the race umpire waved a red flag to halt the contest.