Prime Minister Menachem Begin has charged that Israeli forces in southern Lebanon captured "10 times more Russian weapons than were previously reported by our intelligence."

In an interview Monday with Frank Gervasi of Network News, Begin said that as of July 3, the Israeli Army had taken 4,000 tons of ammunition, 144 armored vehicles including tanks, 12,500 pieces of small arms, 515 heavy weapons, 359 sophisticated communications devices and 795 "optical instruments," among them field glasses and range finders as well as Starlight scopes for night firing.

These weapons alone, Begin said, required a fleet of 10-ton trucks, working day and night for six weeks, to haul them back to Israel. According to military sources, there were enough arms to equip five brigades. "I can now tell you," Begin said, "that only yesterday Sunday we found other arms depots containing fully 10 times as many weapons as we had found before, enough to equip not five brigades, but five or six divisions. We shall need literally hundreds of trucks to evacuate these weapons from Lebanon, where we shall undoubtedly find more arms."

Begin said Israel's intelligence, although reportedly among the best in the world, had greatly underestimated the size of the arsenal which the Soviet Union and other Eastern Bloc countries had placed at the disposal of the Palestine Liberation Organization.

He explained: "Something happened which nobody knew. In fact, the evidence at hand points to a conspiracy, that prepositioning by the Soviet Union of such massive quantities of arms--and I mean modern, highly sophisticated weapons--could only be in preparation for some indeterminate future date to overrun Israel, then Jordan and then Saudi Arabia and other Persian Gulf states. Otherwise, there is no explanation for the quantities of weapons we have found. The PLO itself couldn't have used them, having neither the necessary manpower nor the skills."

Begin would not say, however, whether he believed the Soviet weapons eventually would be used by the Russians themselves or by forces from Cuba, Libya, South Yemen, Algeria or East Germany. All these countries could dispatch large contingents of well-trained fighting men to the Middle East.

Most of the arms, said Begin, were found buried in deep cellars and galleries in bomb-wracked Damour, under piles of rubble and debris. Almost all of the weapons were still in their original packing cases and the inscriptions thereon indicated all had been shipped since the cease-fire which the United Nations Security Council imposed in July of last year.