I need your help and your reassurance.

This is, I know, a time of great uncertainty in large stretches of the Muslim world. The Middle East is in turmoil. Dictators have been toppled, others cling desperately to power, and demonstrators fill the streets and the squares of the Arab world to demand freedom.

I am among those who welcome this struggle for freedom and applaud the voices calling for democracy and human rights in Arab societies. I regret that the American government was slow to encourage the demonstrators, and I am gratified that it has now made clear its support. I also appreciate the complexity of the current situation; in periods of tremendous upheaval, it may be unrealistic to expect the highest standards of civility from all elements of the population.

Nonetheless, I am worried. Even at these difficult moments, there are distinctions that must be made and basic levels of decency that must be maintained. I know that if anti-Semitism makes it way into the fabric of the debates now raging on the Arab street, there will be no turning back; it will endure in the new regimes that will come into being. This has not happened yet in any significant way, but there are worrying indications that it might.

On February 18, a million people crowded into Tahrir Square to celebrate the fall of the Mubarak government. The gathering’s major sermon was delivered by Sheik Yusuf al-Qaradawi, who had returned from exile to deliver his first public address in Egypt in decades.

Qaradawi is a man of great influence in Egypt and throughout the Muslim world. He is also an anti-Semite; his views on Jews are appalling and beneath contempt. In a 2009 televised talk, Qaradawi declared that Hitler was sent by Allah as “divine punishment” for the Jews’ “corruption.” He then expressed hope that Muslims would be the next ones to inflict such punishment upon Jews. In a separate speech, Qaradawi said he yearned for the opportunity before he dies to go to the “land of jihad” and personally shoot Jews.

These comments are not subject to multiple interpretations. They are utterly clear. They were not delivered in an out-of-the-way forum but were broadcast on Al Jazeera, to a television audience of millions.

My friends, you know that I have for many years been a strong advocate for dialogue between American Jews and American Muslims. We have come together often to talk of our common concerns. We have discussed the fact that the values of love, justice and peace are common to both our faiths. We have acknowledged that there are extremist elements in all major religious traditions, including our own. And we have pledged to confront and do battle with the minority of extremists within each of our respective communities, asserting to them and to the world the moderate principles that define our highest ideals.

It seems to me that now is the time for Muslim leaders in America and elsewhere to assure their Jewish neighbors that no matter how events unfold in the weeks and months ahead, anti-Semitism will not be tolerated in any form or for any reason.

Jews are worried because Qaradawi is far from a marginal figure. He is a religious guide to the Muslim Brotherhood and has gained an international audience as the host of the show “Sharia and Life” on Al Jazeera. His words carry great weight. If he gives voice to his well-defined anti-Semitic opinions, it will be impossible to call them back, and they will open the floodgates for others to follow.

You know of my deep and unshakable commitment to the State of Israel, just as I know of your deep commitment to justice for the Palestinians. It is my fervent hope that what is now transpiring in the Middle East will advance our common commitment to a democratic Palestinian state living side-by-side with the democratic State of Israel. Thankfully, this subject has been a matter of the highest priority for the government of the United States, as it remains for me.

Yet as we seek peace, we must not ignore the issue of anti-Semitism. I do not ask you to withhold your expressions of concern and support for the Palestinians. I do ask for the unequivocal rejection of words of hatred and contempt directed at Jews. I do ask for your assistance in sending a message to your co-religionists that anti-Semitism will undermine their societies and corrupt their ideals.

As new, vibrant and — one hopes — fully democratic political systems take shape within the Arab world, I hope that these societies will rid themselves of the hatred of Jews that was so often a tool for the dictators who oppressed them. And I look forward with hope to working with them as partners in a common pursuit of peace.

Rabbi Eric Yoffie is president of the Union for Reform Judaism.