I have seen many riots in France, but none has been as surreal, disturbing or revealing as the mini-riot I saw in the early hours in reaction to the legalisation of gay marriage.

As French riots go, it was not violent. It lasted three hours. Twelve people were arrested; a police inspector was knocked out by a flying paving stone.

A group of about 100 hard right youths chased TV cameramen and press photographers and beat them up calling them “collaborators” and “propagandists”.

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The hard right thugs were supported verbally throughout, and often joined, by up to 500 ordinary-looking, well-dressed, well-off, fresh-faced young men and women from the richest areas of Paris.

Earlier about 5,000 people had gathered peacefully on the Esplanade des Invalides to protest against the approval of same-sex marriage by the National Assembly. There had been much talk from the platform of “dignity”, “peace”, “love” and “family values”.

At about 10.20pm, 30 minutes after the demonstrators had been instructed to go home, a group of 100 to 200 youths, aged 18 to 25, brushed aside the official stewards and attacked the heavily armoured lines of the CRS – the riot police. They hurled bottles, flares, paving stones, fireworks, metal bars and toilet rolls.

They – the thugs and the bourgeois kids alike - chanted “Nous somme chez nous” (this is our home), a standard slogan of the far right National Front. They chanted “Socialist dictatorship”. They sang: “CRS, turn around, the scum are behind you”. In other words, the “scum” were the politicians in the National Assembly at the end of the Rue de l'Université which the CRS were blocking.

Another 400 young people were sitting in candle-lit circles on the grass nearby, singing Catholic scout movement songs about “hope” and “love”.

An illuminated Eiffel Tower shone in the background. A street stall, only 20 metres from the riot, stayed open to serve hot-dogs and cokes to the fiercely nationalistic French kids.

The mob suddenly turned on the TV cameramen and press photographers. One photographer was knocked to the ground and something was sprayed in his eyes. Two of the scout-minded “Catho” girls ran to help him.

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Two other girls – dressed in fashionable scarves, jeans and flat shoes - came up to look. One said: “Pfff. He’s only a journalist.” They ran on to catch up with the mob.

The cross-over between hard right activists and Catho-traditionalist kids fits a pattern which has worried politicians of both Right and Left in recent weeks. The mass gay marriage protests have eroded the increasingly flimsy barriers between the “traditional” and the “hard” right in France; between the National Front and the centre right.

The great majority of the people who demonstrated on Tuesday night and Wednesday morning went home peaceably. But it is inaccurate to claim, as the organisers did, that the trouble came from a “tiny group of hard right infiltrators” or from “CRS provocation”. The riot police were angelic in their discipline and patience.

The mood of the well-heeled kids, arrogant and excitable rather than sincerely angry, fits the mood of their parents. The gay marriage law has crystallised a contempt for President François Hollande which goes beyond his bumbling response to France’s economic woes. This, in part, explains the odd fact that gay marriage is being promoted by a right-wing government in Britain while it is passionately opposed by the right-wing opposition in France.

There are many, including some on the Left, who sincerely object to gay marriage on religious or moral or philosophical grounds. The success of the protests has also been built on a right-wing, bourgeois fury that the Left – the enemy of the “real” France - is in government at all.

There has been much talk in the demonstrations of same-sex marriage as an attack on the “real France”; of a conspiracy by an “ideologically driven” Socialist government to force “a change of civilisation”. The protests have re-awoken old demons – Church versus Republic; “real” France versus “Anti-France” – which weakened the country in the 1890s and the 1930s.

At the same time, the economy is floundering. In Paris, on Sunday week, there will be Hard Left demonstrations against austerity on the same day as right-wing demonstrations against gay marriage. The angelic patience of the CRS may be stretched to the limit.