Ms. Power, who singled out Chancellor Angela Merkel of Germany as among the political leaders who have stood strongly against anti-Semitism, stressed that “rising anti-Semitism is rarely the lone or the last expression of intolerance in a society.” Often, she said, it is “the canary in the coal mine.”

She cited what she said was a recent survey done in the eight European nations where 90 percent of the continent’s Jews live. One in four of the Jews who responded reported having been subjected to attack, and said they felt more insecure than they did five years ago.

Abraham H. Foxman, national director of the Anti-Defamation League, one of the most prominent Jewish groups in the United States, and a member of Ms. Power’s delegation, said Europe was approaching “a moment of truth.” Jewish life returning to places like Germany and Poland, where the Holocaust was planned and largely executed, may not survive the rising insecurity felt by European Jews, he said.

Foreign Minister Frank-Walter Steinmeier of Germany recalled speaking with Elie Wiesel, the Nobel laureate and a Holocaust survivor, after presenting him with a German award this year. He said Mr. Wiesel had told him he would never have believed anyone who tried to tell him in 1945 that he would be fighting anti-Semitism in 2014. “But it is necessary,” Mr. Steinmeier quoted Mr. Wiesel as saying. “The danger is again there.”

There have been several notable attacks on Jews in Europe, including one on a Jewish day school in Toulouse in the spring of 2012, when three children and a teacher were killed, and another on the Jewish Museum in Brussels last May, when four people were fatally shot by a French gunman. In July, an attempt was made to set fire to a synagogue in Wuppertal, Germany, which was rebuilt on the site of a synagogue burned to the ground by the Nazis in 1938.