A move by France's ruling Socialists to ban the far-right National Front from a peace march for Charlie Hebdo shooting victims may backfire, forcing undecided voters to choose between Marine Le Pen and her rivals amid fear of Islamist attacks.

President Francois Hollande's Socialists called this week for a show of national unity on Sunday in support of Charlie Hebdo and media freedom days after gunmen stormed into the satirical weekly's office in an attack that killed 12 people.

But the mood of unity lasted just hours, as the march's organizer said there would be "no room" for the anti-EU, anti-immigration National Front.

Le Pen's party won the highest score of any in European parliamentary elections last May and several polls suggest she could reach the runoff round of a 2017 presidential election.

"The idea of national unity has been totally undermined, and we will face up to the consequences," Le Pen told Le Monde newspaper, calling the move to exclude her from a group of politicians to lead the march a "pitiful political manoeuvre".

Socialist Party leader Jean-Christophe Cambadelis later qualified the FN ban saying "anyone who wanted to and felt concerned" could participate. And Hollande met with Le Pen as well as other party leaders at his offices, as hundreds of police faced off with suspected shooters in an industrial facility northeast of Paris.

But analysts said the damage had been done, with Le Pen set to reap gains from her exclusion and willingness to use strong words to describe the attack, ahead of two local elections this year in which the Socialists are seen suffering further defeats.

"We're at a tipping point," said Francois Miquet-Marty, an analyst for polling agency Viavoice. "French people who felt sympathetic to the National Front but might have been on the fence are now being forced to choose, which may well accelerate a movement toward Le Pen."

2012 Post-Merah Boost

Le Pen said that while Hollande pledged to guarantee her security at the march if she came, he had not explicitly lifted the ban on her participating in the leading cortege.

"I am not going to force my way past security cordons to try to join a march where, quite clearly, the organisers don't want to see us (the National Front)," she said.

Meanwhile Le Pen and allies have rushed to criticise the government's handling of the crisis, both for its security response - suspects were able to escape after the attack, kill two police officers and evade authorities for days - and for the way it has confronted the larger issue of Islamist militancy.

While Hollande blamed forces of "obscurantism" for the attack in a televised address on Wednesday, Le Pen told France 2 TV that "Islamists declared war on France". She also called for tougher policing of suspected Islamists, closing borders to returning jihadi fighters and a return of the death penalty, which has been banned in France since 1981.

In 2012, after gunman Mohamed Merah killed seven people in the southern city of Toulouse, Le Pen said radical Islamists had taken over the suburbs. Her personal approval rating jumped to 32 percent in May, 2012, from 26 percent in March after the killing, according to pollster TNS Sofres.

"She is the only one to put words on things, as they are, without watering down the truth," Sebastien Chenu, a National Front official, told Reuters. "The French obviously know the difference and will vote for the person who is saying things as they are: Marine Le Pen."