Members of the Palestine National Council tonight promised reprisals against those responsible for the death of Lebanese leftist leader Kamal Jumblatt.

The council was locked in heated debate over its policy toward Middle East peace negotiations when the news came that Jumblatt had been assassinated.

Yasser Arafat, leader of the Palestine Liberation Organization, broke into tears as he announced the death of Jumblatt, the Palestinians' closet ally in the Lebanese civil war. As tributes to Jumblatt poured forth, many included threats of reprisal.

The interruption came just at the point at which the council, often described as the Palestinians' parliament in exile, was taking up such critical matters as whether to accept an invitation to a new Geneva peace conference if one was offered, or whether to authorize the executive committee, which is dominated by moderates, to accept without council approval.

The tone of these debates has become perceptibly more pugnacious over the past two days as the Palestinians reaffirmed their enmity to Israel and their anger at the United States. The death of Jumblatt seemed likely to stiffen them still further.

Conference sources said that a hard-line speech this morning - before Jumblatt's assassination - by Fayez Sayegh, a Palestinian elder statesman who is an adviser to Kuwait's mission to the United Nations, had made a deep impression even on those of the nearly 300 delegates who are inclined toward compromise.