The New York Police Department and federal authorities disrupted what officials described as a plot by a member of a pro-Israel terrorist organization to assassinate the leader of one of New York’s most active pro-Palestinian protest groups, according to officials and court papers.

Members of the F.B.I. and the Police Department’s Joint Terrorism Task Force notified the activist, Nerdeen Kiswani, and her lawyer late Thursday that an arrest had been made in connection with an imminent attempt on her life, her lawyer and law enforcement officials said.

The man who was arrested, Alexander Heifler, 26, of Hoboken, N.J., was taken into custody after detectives and federal agents searched his apartment and found eight Molotov cocktails.

A police official said Friday that Mr. Heifler was a member of the JDL 613 Brotherhood, which was inspired by the Jewish Defense League, a pro-Israel group designated by the F.B.I. as a terrorist organization. Representatives of the J.D.L. and its offshoot did not immediately reply to messages seeking comment.

Mr. Heifler was charged Friday morning in a two-count criminal complaint with making and possessing the devices and appeared in federal court in Newark hours later. The complaint did not specify a motive for the planned attack, which was laid out in detail. If convicted, Mr. Heifler could face up to 20 years in prison.

The complaint said the plan had been in the works since at least February, when Mr. Heifler discussed building and using Molotov cocktails for what he described as “self-defense” on a group video call that included an undercover detective.