French President Nicolas Sarkozy declared a day of "national tragedy" in France and schools and religious groups held services of solidarity after a gunman shot dead three children and a rabbi in Toulouse.
As the gunman remained on the loose police confirmed that a gun used in the shooting was the same as that used in three other recent murders in the region.
Presidential candidates suspended their campaigns and cancelled events out of respect.
President Nicolas Sarkozy put south-western France on a "scarlet" terrorism alert for the first time in the country's history.
As children around the country travel to school, he said, guards will be on hand outside all faith-based institutions and Jewish and Muslim buildings.
A visibly moved Sarkozy said from Toulouse: "Barbarism, savagery, hate must not win. The Republic is much stronger than that.
"You cannot murder children like this on the territory of the Republic without being held to account
"I want to say to all the leaders of the Jewish community, how close we feel to them. All of France is by their side."
Socialist presidential candidate Francois Hollande, who is currently leading in most polls ahead of the first round on 22 April, said he wanted to show "solidarity with the families and France's Jewish community".
"This act, whose anti-Semitic nature is as obvious as it is despicable, hits what families hold most dear, their children, and plunges the entire nation into mourning," Hollande said in a statement.
Centrist candidate Francois Bayrou condemned the attack as a "premeditated horror with perverse and hateful intentions".
Far-right Candidate Marine Le Pen also suspended her campaign and said "the whole country is waiting impatiently for this serial killer to be found".
Police said that a man riding a black scooter or motorcycle pulled up outside the Ozar Hatorah Jewish high school at about 8am and fired on adults and children.
A Franco-Israeli teacher and rabbi was shot and killed, as were his two sons aged six and three. Another girl, reported to be the daughter of the school's principal, was also killed.
At least one other person was seriously injured in the shooting, reported to be a 17-year-old boy.
"This man alighted from his moped and, as he was outside the school, he shot at everything he could see, children or adults. Children were chased right into the school," said Toulouse prosecutor Michel Valet.
One witness whose child attends the school described "a vision of horror" in the aftermath of the shooting.
Other parents of children at the school reacted with horror and disbelief outside the school.
The attack has been linked with two attacks on soldiers in the area around Toulouse.
Two French paratroopers were shot in the town of Montauban, near Toulouse, on Friday, when a gunman on a black motorbike pulled up at a cash machine and opened fire, and another soldier in plain clothes was shot at point blank range by an assailant on a scooter on 11 March.
Police said a .45 calibre gun used in Monday's attack was the same as one used in the other two shootings.
With the killer still on the loose Interior Minister Claude Gueant said he will stay in Toulouse for as long as it takes to capture the person responsible.
View Toulouse Shooting in a larger map
A service to remember the victims was held at a local synagogue and there will be a silent march in Paris later this evening to commemorate the event.
Meanwhile across the globe religious and political leaders expressed solidarity and sadness with the people of Toulouse.
The Chief Rabbi of France, Gilles Bernheim, said he was "horrified by what happened this morning in Toulouse". He said that he was "bruised in my body and my soul".
Israel said it was horrified by the shooting, and hoped the French authorities would "shed full light on this tragedy and bring the perpetrators to justice", while Pope Benedict XVI also offered his prayers in a statement.
Jonathan Hayoun, who leads the Jewish students union of France, said in a statement that "anti-Semitic and racist speech has created a climate of insecurity for Jews in France" and called for new security measures to be put in place.
Several other attacks on Jewish schools have been recorded in France in previous years.
In 2009 in Marseille aerosol cans soaked in petrol were thrown at a Jewish school, though nobody was injured, and in 2005 acid bottles were thrown at a Jewish school in Paris.
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