French Prime Minister Manuel Valls has condemned "anti-Semitic" violence that broke out during a protest against Israel's action in Gaza.
Jewish-owned businesses and a synagogue were targeted in the suburb of Sarcelles, just outside Paris.
Shops were looted and 18 people were arrested as youths went on the rampage during Sunday's protest, which had been banned by authorities.
The mayor of Sarcelles said the Jewish community was in fear.
Such an outpouring of violence had never been seen in the suburb before, he said.
Roger Cukierman, head of the umbrella group Crif that represents French Jewish organisations, said Jews were not just afraid, they were anguished.
"What's happened in the past few days is terrible. They're shouting 'Death to the Jews' and attacking synagogues. It's completely out of control", he said.
Pro-Palestinian rallies also took place at the weekend in London, Vienna and Berlin, where an Israeli tourist was attacked by demonstrators.
Anti-Semitic slogans were chanted by some protesters in the German capital. Jewish community leaders have spoken of their shock at the "explosion of evil and violent hatred towards Jews".
A week before the violence in Sarcelles, protesters had tried to target two Paris synagogues, prompting the government to impose a ban on demonstrations in Paris linked to Israel's ground operations in the Gaza Strip.
At least 500 Palestinians and 20 Israelis have been killed.
Mr Valls said nothing could justify attacks on French synagogues or shops.
"What's happened in Sarcelles is intolerable: attacking a synagogue or a kosher grocery, is quite simply anti-Semitism, racism," the prime minister said.
The grocery, which was one of several shops looted on Sunday, had also been targeted in a grenade attack two years ago carried out by a militant cell that was later dismantled by police.
The violence in Sarcelles came after thousands of people in Paris defied the ban on protests on Saturday, with some throwing stones and bottles at riot police, who responded with tear gas.
On a visit to the synagogue in Sarcelles, a mixed area with a large Jewish population, Interior Minister Bernard Cazeneuve rejected criticism of the ban. The ban on the rally had not led to the violence; rather, he said, it was the violence that had prompted the ban.
Authorised rallies also took place in cities elsewhere in France.