NEW YORK, DEC. 4 -- FBI officials and New York City police are investigating a letter warning 10 prominent supporters of Palestinian rights that they would be targets of violence to avenge the assassination of extremist Rabbi Meir Kahane.

M.T. Mehdi, president of the American-Arab Relations Committee who was named on the list, today challenged its sender, saying, "Call us. We will be happy to set up radio-television debates to discuss the issues rather than to suppress each other's First Amendment rights."

The letter, postmarked in New York City Nov. 14 and mailed to a television reporter here, said "things will go boom in the night," according to police sources responding to inquiries about a report in New York Newsday today.

Typewritten in uppercase letters, the one-page note is unsigned but contains a reference to the "Kach Underground Movement" and threatens retaliation when the mourning period for Kahane's death ends, police said. Kahane, shot Nov. 5 after a speech in a midtown hotel here, founded the Kach party, dedicated to expelling Arabs from Israel and the occupied territories.

Also included on the list were Louis Farrakhan, leader of the Nation of Islam; Edward Said, a Columbia University professor of comparative literature and member of the Palestine National Council; Clovis Maksoud, former U.N. ambassador from the Arab League; New York Times columnist Anthony Lewis; Rabbi Arthur Hertzberg, a Dartmouth University professor of religion; Rita Hauser, a New York attorney and U.S. chair of the International Center for Peace in the Middle East, and Rashid Khalid, a University of Chicago history professor and associate director of the Center for Middle Eastern Studies there.

A police source declined to reveal the names of the two others mentioned in the letter but said they have "Middle Eastern-type names," adding, "We have been unable to ascertain specifically who they are."

New York Police Capt. Steven Davis said it is hard to assess the letter's seriousness. "We're treating it as a potential threat, and we've taken measures to insure that people on the list know about it," he said.

Sol Margolis, president of Kach International, an umbrella group of the Kach party, could not be reached for comment.

Mehdi said members of Kahane's Jewish Defense League broke his back in assaulting him in 1974 at a news conference here. But in later years, Mehdi said, Kahane found new strategies that his followers should emulate.

"Those who assassinated Kahane didn't serve their cause, and if supporters of Kahane want to support him, they should be as courageous as Kahane was," Mehdi said.

"Kahane met with me 15 or 20 times on national radio and television discussing and debating the issues. This is how the conflict should be carried out, in a civilized way, an open discussion," he said.

Special correspondent Lauren Ina in Chicago contributed to this report.