The ensuing battle, which lasted for about 90 minutes, resembled one of those ancient football matches played once a year in small British towns. The whole of the main shopping street of Mantes-la-Jolie was the pitch; the National Front leader was the ball. The Frontistes tried to propel him forward; the anti-fascists pushed him back. A few policemen in crash-helmets managed fitfully to get their bodies and sticks in between.
The demonstrators told the Frontistes that they were "Nazis" and "Fascists"; the Frontistes called the demonstrators "drug addicts" and "Aids victims". Blows were exchanged - three times by Mr Le Pen himself.
Le Chef was evidently having a good time. If the game went flat, Mr Le Pen would turn directly towards the mob to stir things up again. On one occasion, amid the pushing and the punches and the flying eggs and the insulting hand-gestures, I found myself standing next to Le Chef himself. "Where is our photographer? Are you sure our photographer is here?" he was asking.
Beside him, his 35-year-old, silver-blonde eldest daughter stood unperturbed in her egg-splattered navy blue suit. According to her mother, who has been estranged from Jean-Marie for 10 years, Marie-Caroline and her two sisters were trained from the age of toddlers to be political animals and to think, and hate, like their father.
"This is nothing to me. I was in the paratroop regiment of the Foreign Legion. I've seen worse than this," Mr Le Pen was saying, rather pompously.
Who says the French parliamentary election campaign was dull and listless? Me, for one. The battle of Mantes-la-Jolie, lively while it lasted, was a microcosm of the campaign: a lot of shouting and screaming, which interested relatively few. At the height of the battle, the good people of Mantes - a pretty town on the Seine 40 miles west of Paris - seemed to just want to get on with their shopping.
Whether they like it or not, their town is pivotal. It is one of the 73 constituencies (out of 577) in which there will be a three-way battle between the centre-right government, the left and the National Front. There are another 80 or so marginal two-way left-versus-government seats which are also important. But it is the "triangular" contests that will do most to destroy the centre-right government, give the left a majority and muddy the plan to create a European single-currency.
The private and illegally published polls have detected a strong leftward momentum since the unexpectedly disastrous government performance in the first round. No one imagines the left will gain a majority of votes; however, the loose alliance of Socialists, Greens and Communists will end up with most seats because the NF will split the right-wing vote.
Mantes-la-Jolie tells the story. Ms Le Pen scored 28.5 per cent in the first round, the incumbent Gaullist, Pierre Bedier 26 per cent and the Socialist, Ms Peulvast-Bergeal, 24.5 per cent.
In the British system, Ms Le Pen would have been elected. In the French two-round system, with most of the votes of the eliminated Communist, Green and fringe-left contenders going to Ms Pelvast-Bergeal, the Socialists will probably win. The same thing will happen in between 50 and 60 of the 73 three-way constituencies.
So you will see why it is difficult to talk confidently of France turning left today, or France voting against European Monetary Union, or France doing anything coherent at all. The most striking feature of the first- round was how fragmented it was. Just 12 candidates were elected without a run-off, fewer than ever before. There were more candidates running than ever before; more small parties; and a smaller share of the vote for the mainstream parties. The NF score - 15 per cent - was the highest ever in a national election and close to the point where they can permanently disrupt the traditional right.
The oddest, and most emblematic, result in France was in Grasse, the lavender capital of Provence. The only candidates left standing after the first round were a Frontiste and a Green: Grasse will go Green today.
However, the election is coherent in one sense: it will be a rejection and humiliation of President Jacques Chirac. Francois Mitterrand was forced into "cohabitation" with the right after five difficult years, but Mr Chirac has been repudiated after only two. It could be argued that his policies - to reform and shrink the welfare state; to support the single currency at all costs - were right for France. But these were not the policies he sold the electorate in the presidential election in 1995. He called this election early to catch the left and the NF napping, but it was he who ended the election looking, frankly, incompetent, without a Prime Minister or a clear policy. That may still leave the Socialist leader Lionel Jospin, good and competent man though he is, Prime Minister by default.
There is no great enthusiasm for the return of the left. Unlike Tony Blair, Mr Jospin does not inherit a benign economy. The interlocking problems - the budget, unemployment, EMU, sluggish growth - are daunting. A former socialist minister told me last week, after surveying the wreckage of the first round: "If the right wins it will be impossible for them to govern. If we win ... it will be a nightmare."