(CNN) -

A mob, wielding baseball bats, broken bottles and knives, swarms a Paris synagogue. Violence erupts at a pro-Israel rally in Los Angeles after a demonstrator reportedly stomps on a Palestinian flag. Phone calls and text messages threaten a Palestinian-American who organized a protest in Atlanta. A trending Twitter hashtag says Hitler was right.

As missiles and rockets fly in the Middle East, tensions are boiling over around the world between activists at demonstrations on both sides of the Israeli-Palestinian conflict. Plenty of protests have been peaceful, but not all of them.

On Monday, the Anti-Defamation League warned Jewish institutions to step up security in light of violence and anti-Semitic expressions at what it described as anti-Israel rallies across the United States and around the world. An ADL website tracking recent protests listed events in New York; Washington; Dallas; Portland; and Tempe, Arizona.

"The tenor at some of the anti-Israel rallies has been extreme," the ADL said, "with protesters chanting 'Death to Israel' and other hateful messages and slogans."

In France, where anti-Semitism has flared up in recent years, some warn that hostilities have entered a different realm.

"The level of danger is very new," said Serge Benhaim, who was trapped for hours inside a Paris synagogue on Sunday. "Today and tomorrow for the Jewish people in France is fully different from what it was yesterday."

In the United States, too, Aysha Abdullatif says she's sensed something is changing.

After organizing a pro-Palestinian protest in Atlanta this month, Abdullatif said she started getting threatening phone calls, text messages and social media posts accusing her of supporting terrorism. It's the first time she's felt personally targeted after years of activism.

"People are getting really fanatical. ... I've never seen it get this ugly," Abdullatif said.

'It looked like a war'

A 17-year-old Jewish girl reported that she was grabbed by the jaw and pepper-sprayed in the face on a Paris street the day Israel launched its latest operation in Gaza. She told police her attacker called her a "Dirty Jew," and said, "Insha'Allah, you will die," according to the National Bureau of Vigilance against Anti-Semitism, a French watchdog organization.

A local chapter of the Jewish Defense League, a far-right Jewish group, bragged on social media the next day about fighting with anti-Israel demonstrators.

"We were 30 facing 200 supporters of Hamas. And yet all will remember our visit ... especially the 6 wounded on their side," the group posted on Twitter.

A French watchdog organization, meantime, has since reported telephone death threats against Jewish merchants. Synagogue-goers in the Belleville neighborhood of Paris were recently greeted by demonstrators who screamed "Death to the Jews," and a firebomb was tossed at the entrance of another synagogue in the northeastern suburb of Aulnay-sous-Bois, the National Bureau of Vigilance against Anti-Semitism reported.

And then, on Sunday evening, a perfect storm brewed. Just as a community gathered in Paris' Don Isaac Abravanel Synagogue to pray for peace, thousands of demonstrators marching in support of Palestinians finished up nearby, Benhaim, the synagogue's president, said. A fraction of those demonstrators broke off with other plans.

Aline Le Bail-Kremer, 36, lives across the street from the synagogue and said she saw -- and heard -- them coming.

"From my windows, I saw two groups (around 100 persons), from the two sides of the street, converging [at] the synagogue," she wrote in an e-mail to CNN late Monday.

They carried baseball bats, she said. They threw chairs and tables, taken from nearby cafes, and headed toward the entrance gate. And then, she said she heard them scream, "Death to the Jews."

From inside, where he'd gathered with about 400 others, Benhaim said he saw men outside brandishing broken bottles and knives. The synagogue president, who CNN spoke to Monday night, also said he heard cries of "Jews to the oven" and "Allahu akbar!"

A small band of security guards managed to block entry, Benhaim said. Young Jews, some affiliated with the Jewish Defense League, also stepped into the fray -- spewing their own vitriol, Le Bail-Kremer said.

The Jewish Telegraphic Agency, a wire service for global Jewish news, reported that "at least three Jews were taken to the hospital as a result of the clashes." This chaos went on for about 40 minutes, Le Bail-Kremer said, before police arrived in droves.

"The scene was very violent, with terrifying and anti-Semitic slogans," said Le Bail-Kremer, who happens to be involved with SOS Racisme, a French anti-racism organization. "I was very, very anxious and shocked. It looked like a war."

Trying to 'turn the other cheek'

As she stepped forward to make closing remarks at a pro-Gaza demonstration she organized over the weekend in Atlanta, Abdullatif said she saw the crowd turn the other way.

Across the street, she said, there were two men with Israeli flags who discharged pepper spray toward the crowd.