France's far-right National Front (FN) party is "at the gates of power", Prime Minister Manuel Valls has warned.
He said his Socialist government had to act and speak "differently" to counter the threat from FN.
His comments came after opinion polls suggested that FN leader Marine Le Pen could beat incumbent Francois Hollande in the 2017 presidential election.
Meanwhile, a far-right rally held in Calais, in the north, demanded that the port city be "saved" from migrants.
Riot police kept the protest march away from a smaller group of left-wing activists who were defending the migrants.
Large numbers of migrants have arrived there in recent months. Many of them have been trying to cross illegally into the UK, a country they see as providing generous benefits to refugees.
French police have boosted their presence in the port after dozens of migrants tried to storm a ferry bound for Dover.
"In France, the extreme right of Marine Le Pen is at the gates of power," Mr Valls said at a gathering of fellow left-wing politicians in Bologna, Italy.
"We have to act differently. We have to speak differently. In order to be listened to and to be heard.
"And I, as a man of the left, will never be able to resign myself to that because it will be the weakest who will be the first to suffer. And it will also be a terrible, perhaps fatal, blow to Europe," Valls added.
He was referring to opinion polls suggesting that Ms Le Pen would easily make the run-off in the 2017 elections - and could win should her rival in the decisive round be Mr Hollande.
Mr Valls' warning comes as President Francois Hollande's ratings remain at an all-time low, and the government faces a confidence vote later this month.
Mr Hollande has vowed to kick start growth and create jobs, but the economy has floundered, prompting last month's revolt against austerity by three ministers.
Mr Hollande replaced them and named a new cabinet in what was described as an emergency reshuffle.
In July, the number of people looking for work in France rose to a new record of 3,424,400.