* Pro-Palestinian protesters defy ban on rally in Paris
* Demonstrators clash with police, 38 arrested
* Rallies in rest of France, London proceed peacefully (Adds arrests)
By Philippe Wojazer
PARIS, July 19 (Reuters) - Pro-Palestinian protesters clashed with police in Paris on Saturday as they defied a ban on a planned rally against violence in the Gaza strip.
A Reuters photographer said demonstrators in northern Paris launched projectiles at riot police, who responded by firing teargas canisters and stun grenades.
Demonstrators also climbed on top of a building and burned an Israeli flag. At least one car was set on fire.
A police spokesman said that 38 demonstrators had been arrested by early evening and that the clashes were dying down.
However, dozens of police trucks were seen rolling into the narrow streets of the historically Jewish Marais neighbourhood where French media said groups of protesters had assembled.
President Francois Hollande earlier said he had asked his interior minister to ban protests that could turn violent after demonstrators marched on two synagogues in Paris last weekend and clashed with riot police.
"That's why I asked the interior minister, after an investigation, to ensure that such protests would not take place," he told journalists during a visit to Chad.
In defiance of the ban, large crowds gathered in northern Paris chanting "Israel, assassin" until they were dispersed by tear gas.
Peaceful rallies were also held in more than a dozen other cities, from Lille in the north to Marseille in the south.
"This ban on demonstrations, which was decided at the last minute, actually increases the risk of public disorder," the Greens Party said in a statement. "It's a first in Europe."
Interior Minister Bernard Cazeneuve justified bans in Paris and the Mediterranean city of Nice by saying the security risk was too great, prompting outrage from left-wing and pro-Palestinian groups.
Foreign Minister Laurent Fabius responded to criticism France was biased in favour of Israel, which sent ground forces in on Thursday after 10 days of air and naval barrages failed to stop rocket fire from Gaza.
"In no way does this mean that the French government has taken a position against the Palestinians," he told journalists during a visit to Jordan.
Elsewhere in Europe, a man set off a security alert in Geneva when he stopped a tram to retrieve bags that included a book with a radical Islamist image in it, police said.
The alert coincided with a demonstration against Israel's assault on Gaza that drew some 300 protesters to the front of the U.N. European headquarters in the Swiss city.
In London, thousands of pro-Palestinian demonstrators marched peacefully clutching Palestinian flags and banners reading "Stop the bombing" and "Free Palestine" before congregating outside of the Israeli embassy.
The Israeli-Palestinian conflict has contributed to growing tensions between France's Muslim and Jewish populations, both of which are the largest in Europe.
In the first three months of 2014 more Jews left France for Israel than at any other time since the Jewish state was created in 1948, with many citing rising anti-Semitism as a factor. (Reporting by Philippe Wojazer; Additional reporting by John Irish in Jordan and Costas Pitas in London; Writing by Nick Vinocur; Editing by Alison Williams, Sophie Hares and Sonya Hepinstall)