Paris (AFP) - The Bastille Day attack in Nice -- the third terrorist massacre to hit France in 18 months -- has sparked anger and racism, putting further strain on an already tense political atmosphere.
"What is clear is that it has brought to the surface the fault lines which were always there but which were not so apparent," sociologist Michel Wieviorka told AFP.
"When people can boo the prime minister and talk in an openly racist way," norms have been shaken, he argued.
President Francois Hollande has said "anger is legitimate" in the wake of the July 14 attack in Nice, in which Tunisian Mohamed Lahouaiej Bouhlel killed 84 people as he rammed a truck through a crowd watching fireworks for France's national day.
"After such a shock, anger is legitimate, because our compatriots were killed, and innocent people hit," Hollande said.
"But it cannot degenerate into hate and suspicion."
In Nice, rubbish has been heaped on the spot on the Promenade des Anglais where Bouhlel -- who the Islamic State group has claimed as one of its "soldiers", though investigators have found no direct proof of allegiance -- was finally shot dead.
Chalked on the ground next to the heap of rubbish is the word "coward".
But some of the anger has also been directed at Muslims -- even though, as the city's mayor said, they were "probably the worst hit by this attack". According to local imams, about a third of the dead were Muslims.
Even at a minute's silence in the Riviera city for the victims on Monday, anger and hatred boiled over as a video which has been viewed more than six million times on Facebook makes clear.
"Go back to where you come from," a middle-aged man shouted at a young woman of North African origin. "I was born in France, sir," she replied. "Where should I go?"
"You are a shame on France," he shouted back, with some in the crowd supporting him, as a policeman stepped in to intervene.
- Poisonous atmosphere -
At the same ceremony, Prime Minister Manuel Valls was a target for sustained booing, with people shouting out that he should resign.
"Violence is lurking a bit everywhere in France," said Wieviorka, referring to months of clashes at nationwide street protests against changes to the country's labour laws.
"We are experiencing something which is being made worse by the political context with presidential (and parliamentary) terms coming to an end. All this creates conditions which maximise the impact of terror," the sociologist added.
In contrast to the huge street protests that united the nation after the Charlie Hebdo and kosher supermarket killings in January 2015, the Nice attack has poisoned the political climate.
Nine months from May's presidential election, the right and far-right opposition parties are lambasting the ruling Socialists, whose popularity has sunk to a record low.
Interior Minister Bernard Cazeneuve has come under intense pressure to resign, claiming that he was the victim of an "undignified" campaign to discredit him orchestrated by Nice's former right-wing mayor Christian Estrosi.
The row became even uglier Sunday when a senior policewoman said the minister had pressured her to alter a report into security in Nice on the night of the attack.
Cazeneuve categorically denied the allegations and threatened to sue.
Her claims came only days after local authorities resisted a request to wipe "shocking" security camera footage of the carnage which prosecutors said they feared might leak out.
- Fears over resilience -
The Nice massacre was the third mass killing in France in 18 months after the Charlie Hebdo attack and November's jihadist carnage in Paris.
In all, 230 people have been killed and several hundred injured, a toll never before seen in peace time in France's modern history.
Top security officials recently admitted to a parliamentary inquiry that they were worried about the repeated impact of such killings.
"It puts the resilience of French society into question," said Bernard Bajolet, head of the foreign intelligence service, the DGSE.
He said France had to strengthen itself "morally" for a struggle over a very long period.
"This is reminiscent of Italy's 'Years of Lead'," he said, referring to nearly two decades of political violence from the 1960s between far-left and far-right groups.
His opposite number who runs the DGSI domestic intelligence service, Patrick Calvar, said he was confident that the authorities would "win out against terrorism", but was "a lot more worried about the radicalisation of society and where this would lead".
He feared a slide into "a confrontation between the far-right and Muslims; not the Islamists but the actual Muslim world".
"I don't at all think we are talking about a civil war. On the other hand I see a country moving to the right and which is looking toward to the extreme right," Wieviorka added.