Israel's departing defense minister, Ezer Weizman, bitterly denounced Prime Minister Menachem Begin and his government tonight, accusing him of throwing away an opportunity to bring peace to the Middle East.

Weizman, who announced yesterday that he was quitting the government because of deep differences with its policies, said in his letter of resignation today that "we are marking time instead of marching toward peace."

He accused Begin personally of "missing a historic opportunity to provide leadership to the Israeli people who had given him their trust."

The letter drew an immediate sharp response from Begin, who said he was "shocked by the accusations" and added that "there cannot be any harsher slander than such charges."

The bitter exchange appeared likely to make even more difficult Begin's task of choosing a successor to Weizman who can win acceptance from all factions of his ruling coalition. In the long run, it could affect Begin's capacity to govern a politically divided Israel.

Begin, who made clear that Weizman's letter could have serious international repercussions, argued that if the talks with Egypt over Palestinian autonomy are currently stalled, this is because "the Egyptian proposals over Jerusalem and other subjects endanger the security and future of Israel and are in contradiction to the Camp David agreements."

The autonony statute was to have been completed today, according to the Egyptian-Israeli accord of last year, but the date arrived with little notice given it.

Begin reminded Weizman of his having personally rejected the proposals concerning the security of the proposed autonomous entity as presented by Egypt during the last round of talks.

Political commentators here said Weizman's repudiation of the Israeli position in the autonomy talks -- the same issue that caused foreign minister Moshe Dayan to resign seven months ago -- may shatter any hopes that the government had of winning international support for its position.

Although Begin promised his Herut faction in parliament today to appoint a new defense minister "very soon," his hopes of ending the Weizman crisis within the next few days may be thwarted. Begin's coalition partners, in a series of caucuses today, intimated that they may exploit the crisis to gain additional Cabinet seats.

Begin's own candidate for the defense post is reportedly Moshe Arens, 54, an American-educated aeronautics professor at the Haifa Technical Institutes. Arens, chairman of the Defense and Foreign Relations Committee of parliament, belongs tto the right wing of Begin's Herut Party. Like Foreign Minister Yitzhak Shamir, Arens voted against the Camp David agreementns.

Another contender is Agriculture Minister Ariel Sharon. However, the opposition to Sharon by two coalition paartners, the Liberal Party and Deputy Prime Minister Yigal Yadin's Democratic Party, may be more vehement than their opposition to Arens.

Another complication for Begin may be the threat of the chief of staff, Lt. Gen. Rafael Eitan, to resign. According to the afterrnoon newspaper Maariv, Gen. Eitan asked for an urgent meeting with the prime ministerr to explain his opposition to cuts in the defense budget, the issue that precipitated Weizman's resignation.