Noting that anti-Semitism was among the oldest forms of prejudice known to mankind, Secretary-General Ban Ki-moon said: "the UN had a duty to speak out against it, as efforts to build a world of mutual understanding were being tested today by rising extremism and barbaric acts."
"The poison of hatred was loose in too many places. Jews remained targets, as did Muslims and many others," he said.
"Our responses must avoid perpetuating the cycles of demonisation and playing into the hands of those who seek to divide," the Secretary-General said.
Similarly, criticism of Israeli action should not be dismissed as anti-Semitism, he said.
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The fight against the scourge was a "fight for all of us", he said, as anti-Semitism was "inseparable from the wider quest for peaceful coexistence and human rights".
The assembly met at the request of 37 western countries, including the US who requested the UN to address the "alarming outbreak of anti-Semitism worldwide."
It was an informal meeting, attended by about half the 193 member states, so no resolution could be adopted.
French philosopher Bernard-Henri Levy told the 193-nation assembly that the world must confront "the renewed advance of this radical inhumanity".
Ron Prosor, Israel's Permanent Representative to the UN, said years after the Holocaust had seen the murder of six million Jews, violent anti-Semitism was "casting a shadow" over Europe, including attacks, hate-mongering and killings.
He noted that anti-Semitism also existed at the UN, including last autumn when delegations accused his country of behaving like the Nazis and creating a Holocaust in Gaza.
However, action was needed, not statues or commemorations, he said, emphasising that the days when Jews were the world's victims were over.