French President Jacques Chirac has attended a memorial for a young French Jew whose killing shocked the nation.
Mr Chirac (C) led religious and political leaders at the service
Ilan Halimi was held for three weeks then found naked with horrific injuries, dying on the way to hospital.
The suspected head of the kidnap gang, Youssouf Fofana, has been held in Ivory Coast and his extradition is expected to be completed shortly.
French ministers have suggested the crime may be anti-Semitic but Ivorian police say Mr Fofana has denied this.
President Chirac, his wife Bernadette, Prime Minister Dominique de Villepin and the head of the opposition socialists were joined by leaders of the Jewish, Christian and Muslim religions for the evening service at the Grand Synagogue in Paris.
Mr Halimi's mother and two sisters were also there and spent time talking to the president.
France's Grand Rabbi Joseph Sitruk said: "From now on, in France, there will be the period before Ilan and that after Ilan.
"Today, I ask all French citizens to all stand up as one man and shout loud and clear, 'Enough is enough'," he said.
Mr de Villepin said: "National solidarity is being expressed."
The prime minister had earlier told a meeting of Jewish community leaders that the judge handling the case was investigating leads pointing to an anti-Semitic attack.
"We will do everything we can to arrest the authors of this barbarous crime and bring them to justice," he said.
Lured by woman
The prime minister's office said he had spoken to Ivory Coast President Laurent Gbagbo and hoped Mr Fofana might be returned very soon.
Police are investigating anti-Semitic links to Mr Halimi's killing
Mr Fofana, 26, was detained in the city of Abidjan on Wednesday.
French police say he headed the gang that kidnapped, tortured and killed Mr Halimi, a mobile phone salesman. A woman was used to lure him and other victims, police say.
More than 1,000 people, mostly Jews, marched in the capital on Sunday to condemn the killing.
Interior Minister Nicolas Sarkozy said four out of the gang's six previous targets were Jewish, who were chosen because of the belief that "Jews have money".
But police in Abidjan told the AFP news agency Mr Fofana "denies any anti-Semitic dimension".
Other police sources there told Reuters: "He said there was no plan to kill him. He only wanted the money."
Thirteen people have been indicted over the gang's activities, 11 of whom are in custody, French police say.
Six other suspects are being held, one of them in Belgium.
Mr Halimi was dumped near the Sainte-Genevieve-des-Bois train station near Paris on 13 February.
His family were sent ransom demands by e-mail and text messages from the kidnap gang, whose leader called himself "brains of the barbarians".