"We're scared of what might happen," Amel, 34, told CNBC ahead of the final round of voting in France's snap election this weekend.

The vote is being closely watched by all quarters of French society to see if the nationalist, anti-immigration National Rally (RN) builds on its initial win in the first round of voting, or whether centrist and leftwing parties have been able to thwart the party's chances of entering government.

"It's a very, very tense time. And it's the first time that the far right is winning at the first turn [the first round of the ballot]. So it's a very big deal," Amel, a therapist who said she will vote for the leftwing New Popular Front, added.

"We are very anxious and we are trying to get everyone to vote, trying to tell people who don't vote to go and vote, and to try to convince people who vote for the extreme right that they are not a good answer [to France's problems]."

France's far-right RN rejects the "extremist" label, saying it stands up for French values, culture and citizens at a time when many are fed up with France's political establishment that's been led by President Emmanuel Macron since 2017.

But RN's opponents and critics warn France is on the brink of a political catastrophe if an overtly anti-immigration, nationalist and euroskeptic party wins a majority in this snap election called by Macron after his party lost heavily against the hard-right in European Parliament elections in June. Prime Minister Gabriel Attal has said French voters now have a "moral duty" to halt the party's advance.

For young, left-leaning voters like Amel, RN's surge in voter polls, and the fact it won the most votes in the first round of the election last weekend, are worrying developments that make them fear for France's societal cohesion.

"I am worried about the country's future. I think it's getting worse and worse," Amel, who preferred to only give her first name due to the sensitive nature of the situation, said. "It's going be like a kind of civil war. I hope it will not reach that, but people will just not mix anymore and will be scared of each other. And this is very scary."

The snap election has thrown the country's political polarization into sharp relief as polls ahead of the final round of voting on Sunday imply a deeply divided nation.

The first round of the election resulted in the far-right RN winning 33% of the vote, with the leftwing New Popular Front (NFP) garnering 28% and the coalition of parties supporting Macron (Ensemble, or Together) winning 20% of the vote.

Since the results of the first ballot, parties on the center-right and left have gone all-out to prevent RN's advance in the second ballot, aiming to prevent a parliamentary majority for the party at all costs. Joining forces in a so-called "Republican Front," centrists and leftwing parties have withdrawn candidates in many constituencies where one of their candidates was better placed to beat the RN.

By offering voters a starker choice and fewer options, the anti far-right front hopes that the electorate will vote for the non-RN candidate. Whether it will work remains to be seen and analysts point out that French voters might not take kindly to being directed how to vote, or who to vote for.

The final result on Sunday evening — the outcome of a snap election Macron did not need to call — will show just how hard it could be to find a consensus in national politics and government going forward.

How the nation will react to the result is also uncertain. France is no stranger to civil unrest given the widespread "Yellow Vest" anti-government movement of recent years, and street protests since the first round of voting on June 30.

France's Interior Ministry appears to be preparing for more trouble after Sunday's poll, reportedly ready to deploy around 30,000 officers across France on Sunday night amid fears of violence after polls close. Interior Minister Gérald Darmanin is reported to have said 5,000 police would be on duty in Paris and its surrounding areas to "ensure that the radical right and radical left do not take advantage of the situation to cause mayhem."

France's police force has, at times, been accused of being heavy-handed with demonstrators during previous periods of unrest, firing water cannon and tear gas at "yellow vest" demonstrators in 2019.

A member of the gendarmerie, France's military force in charge of law enforcement and public order, told CNBC that the "French elections are a mess" and that the "public divide has rarely been so flagrant in France."

"People's opinions are becoming more and more divided and this is felt in everyday life," the gendarme, who asked to remain anonymous due to the nature of his job, told CNBC.

The officer — a father of three who's in his 40s, and a right-leaning voter — said the polarization in French society was "very worrying, but unfortunately normal with the 'diversity' of our society."

"More and more people with different values and educations are being forced to co-exist, and this clearly doesn't work," the officer, who works in Bordeaux in southwestern France, said.

"I am worried about the country's future, because we are too generous to people who aren't willing to integrate and contribute to our society, this can not last."

The police officer said he expected civil unrest after the vote, whichever party gained the most votes.

"There will be civil unrest whoever is elected, this is France and the people speak their mind."

Political experts agree that the current febrile atmosphere of French politics, and antagonism between the main bodies of voters, are the ingredients for further civil unrest.

"You've got here all the recipe for a super-polarized political scene and that, of course, translates into civil society as a whole," Philippe Marlière, professor of French and European politics at University College London, told CNBC.

"If you've got only 33-34% of people voting for the far-right it means the rest is wary of that, or completely opposed to it, so that will translate on every level of politics — institutional politics, party politics, the National Assembly, but also in society. You will have a very polarized society in which younger people, ethnic minorities, women, and in particular feminists, would be very worried," he said.

Marlière did not discount the possibility of violence on the streets if a far-right party was elected to government. "We're not there yet. But if there are very unpopular, very antagonizing and very hostile policies to some groups, there will be demonstrations on a scale that you have unrest in the street," he said.

Like other hard-right parties in Europe, the National Rally has tapped into voter insecurities regarding crime, immigration, national identity and economic insecurity. RN's 28-year-old leader Jordan Bardella has told voters he will "restore order," curb immigration and tackle delinquency but he and party figurehead Marine Le Pen have rowed back on some of their more strident promises and rhetoric, back-pedaling over taking France out of NATO, for example, and moderating the party's traditionally pro-Russian stance.

Bardella said he would still support the sending of arms to Ukraine but not the deployment of ground troops, as Macron suggested was a possibility.

It's uncertain how many of National Rally's policies would be enacted even if the party made it into government. The "Republican Front" also appears confident ahead of the second round of voting that its strategy to hurt the RN's vote share is working.

An opinion poll published by Ifop on July 3 suggested voters might tend toward a centrist pro-Macron or leftwing candidate rather than the RN candidate if that is the choice they are presented with on the ballot paper on Sunday. If the choice was between a far-left and far-right candidate, however, the picture was more nuanced, showing a split vote.

Analysts predict that RN is less likely to be able to achieve an absolute majority of 289 seats in the 577-seat National Assembly, but is still likely to gather the most votes, creating a hung parliament scenario and headache for Macron and uncertainty for France's political and economic outlook.

"The political landscape is in turmoil and can't really work any longer, at least not by the old rules," Ipsos analyst Mathieu Doiret told CNBC Thursday.

"We are in a situation so far from our traditions and political habitus that it's very difficult to adapt to this new situation for every stakeholder."