A mysterious substance that has caused illness among hundreds of Arab children in the past week set off disturbances in the West Bank town of Jenin today as authorities sought clues to the origin of the poisoning.

There were no injuries reported in the disturbances, but Israeli occupation authorities imposed a curfew on Jenin this morning after Palestinian youths in the town threw stones at Israeli vehicles.

Almost all of the victims of the poisoning were girls, students at Moslem schools for girls in the Jenin area. There were conflicting accounts of the number of victims and of the likely origin of the illness, but both Israeli authorities and Palestinians agreed that it appeared to be the result of a deliberate act by unknown persons.

Palestinian sources in Jerusalem, quoting Arab leaders in the Jenin area, said 480 children were affected by the illness and blamed the incident on Israeli authorities and Jewish settlers in the West Bank.

Maj. Amid Zaid, the Israeli military commander in Jenin, said 307 children had become ill and that "local Arab extremist elements were most likely behind the incident."

No one was reported to be critically ill as a result of what authorities described as a "poisoning," but more than 100 children were treated in hospitals. The symptoms of the illness included headache and nausea, and medical officials said a few of those treated appeared to be victims of mass hysteria.

Israeli civil administration officials said the illness appeared to have resulted from "some kind of poisonous gas" spread through the air. There were reports that a "yellow dust" had been found on a window sill of one of the schools and was being analyzed.

Reuter reported that authorities said curtains in the school classrooms appeared to have been sprayed with a chemical whose fumes caused the poisonings. Specimens were sent to a Health Ministry laboratory to determine what chemical was involved.

A civil administration official said investigators believed the illness resulted from a "deliberate attempt to incite the population" just before the annual observance on Wednesday of the Land Day holiday by Arab residents of Israel and the West Bank.

Israeli officials said a leaflet was found in a boys' school in the Jenin area warning the students that if they did not demonstrate on Land Day then they too would fall victim to the illness.

Land Day, often the occasion of violent disturbances, was begun in the early 1970s by the Arabs of the Galilee region to protest government confiscation of Arab land. Israeli authorities are bracing for a possible wave of unrest both in the West Bank and the heavily Arab sections of Israel as the date for the observance approaches.

The first cases were reported last Monday when 32 girls at a village school near Jenin became ill. The number of victims increased yesterday and today as other schools also were affected. Zaid said cases of the illness occurred at six schools in the Jenin area.

All schools in Jenin were closed today and are expected to remain closed at least until after the observance of Land Day.