The chairman of the House Foreign Affairs Committee delivered a testy lecture to Israeli Prime Minister Menachem Begin yesterday and later said he is positive that Israel violated U.S. law in using American arms to invade Lebanon.

Rep. Clement J. Zablocki (D-Wis.) told Begin that his Wisconsin constituents had upbraided him for supporting Israel too long, and one group told him bluntly his hands "are as bloody as those of Israeli soldiers."

Those remarks, one of the strongest congressional criticisms of the Israeli invasion, were made during a reception for the prime minister on Capitol Hill.

Zablocki, who as Foreign Affairs chairman oversees billions of dollars of aid to Israel, until yesterday had maintained a neutral position on the war in Lebanon. Although there have been scattered criticisms of the Israeli attack, the congressional response generally has been muted.

Zablocki told Begin that a main complaint he received was that Israel "flouted" American law in using U.S.-supplied equipment for offensive purposes in going after Palestinian positions in southern Lebanon.

Later, he told reporters that he shared his constituents' view: "There is no doubt in my mind. The law is very clear--it the equipment is intended for defensive purposes."

Zablocki said that it would be legal to use those arms in "hot pursuit" of an enemy retreating from Israel's borders, but he said "in the case of Lebanon, Israel went too far."

He also said congressional support for Israel has been "eroded" by the invasion, an Israeli air attack last year on a nuclear reactor in Iraq and the occupation of the Golan Heights.

The question of possible Israeli misuse of American weapons has cropped up several times and several members of Congress are probing reports that Israeli forces used cluster bombs in Lebanon in violation of a special agreement between the two countries.

Zablocki pointed out yesterday that nothing can be done unless the administration makes a formal finding that those weapons have been misused.

Yesterday, the Senate Foreign Relations Committee was told there had been no Israeli response to questions about a newspaper report that cluster bombs had been used. Deputy Assistant Secretary of State Wat T. Cluverius said the Israeli government was asked for information last Wednesday or Thursday by the American Embassy.

The Senate committee voted tentatively to approve $50 million in emergency aid for war-torn Lebanon.

Committee Democrats called President Reagan's proposed $20 million program inadequate and indicated they may attempt to raise the total even higher than $50 million when the bill gets to the Senate floor. "Twenty million is an embarrassing figure considering the damage that has been done," said Sen. Paul E. Tsongas (D-Mass.).

Most of the money will be spent through international and private relief organizations that are attempting to provide food, medicine and shelter to thousands of Lebanese caught up in the fighting.