An Arab hurled two hand gernades into a group of about 40 Jews, most of them teen-agers waiting for a bus to take them to summer camp, killing one and injuring 20, police said.

David Kuhan, 15, of Paris, was killed when the grenades exploded this afternoon in the crowd outside the Agoudath Israel cultural center near the downtown area.

Police said the man they arrested after a chase by witnesses carried a Moroccan identity document but was believed to be Lebanese. Her was alsocarrying a pistol and several magazines of ammunition but did not resist arrest.

When asked his motive, the suspect replied in English: "You know why," which police said was a reference to the Arab-Israeli conflict. Antwerp has a large Jewish community known for its active support of Israel.

The group included Jewish children from Austria, Britain, France and the Netherlands who had come to Belgium for a trip to a vacation camp in the Belgian Ardennes.

A 13-year-old boy from Antwerp was hospitalized with a critical brain injury. Six other persons, including a pregnant woman of 27, were taken to the hospital with serious grenade wounds. The children in the group were between 10 and 14 years of age.

Most of the children belonged to families engaged in the diamond business. Most of the craftsman and dealers in daimonds here are Orthodox Jews.

Police searched the area more for suspects, but spokesman quoted the Arab as saying he acted along.