The Herut Party, the nucleus of Israel's right-wing Likud bloc, tonight opened the first national convention it has ever held without the domination of Menachem Begin, the former prime minister.

On the surface, the convention opened in a spirit of unity, with delegates singing nationalistic songs of the revisionist Zionist Betar movement founded by Begin. There were undercurrents of reconciliation among the rivals for the party leadership.

But Begin, who went into seclusion in 1983 after resigning as prime minister, did not join the nearly 1,900 party delegates at the convention's opening session nor outwardly attempt to influence its outcome, thereby ensuring a continuation of the fierce behind-the-scenes struggle among three rivals for party leadership: Foreign Minister Yitzhak Shamir, Deputy Prime Minister David Levy and Trade Minister Ariel Sharon.

As the major candidates filed into Jerusalem's convention hall, their supporters began their chants for the competing party figures, with Levy's supporters loudly intoning, "David Levy, melach Yisrael" -- king of Israel.

Party leaders said they expected the power struggle to begin in earnest Monday, when the convention moves to Tel Aviv and key committee leadership posts are parceled out in caucus bargaining.

Leadership of the "national unity" coalition government is not at issue in the convention, since Levy and Sharon have agreed that Shamir will succeed Prime Minister Shimon Peres of the Labor Party under the rotation of the premiership scheduled for next fall.

However, the three-day convention, the first to be held by the party in seven years, will determine the balance of power in its governing Central Committee, which will become crucial in 1988 when Shamir, Levy and Sharon make their bids for the top position in parliamentary elections. The showdown could come sooner if the coalition government collapses and early elections are called.

In addition, the leader of the Herut Party is also the effective leader of the Likud bloc, since Herut is by far the largest party in it.

Party strategists said it probably will not become clear which of the rivals has gained the upper hand until Monday's selection of chairmen of three key party committees: the steering committee, which will determine the composition of the new Central Committee; the mandates committee, which can add more delegates on technical grounds; and the party presidium, which oversees the convention.

Levy and Cabinet member Moshe Arens, a Shamir ally, are contesting for the steering committee, which will also draft the next list of candidates for parliamentary elections, while Sharon is expected to battle for the mandates committee.

While ideological differences among the rivals are not expected to play a central role in the convention -- as they did in the 1979 convention, between supporters and opponents of the Camp David peace accords -- Sharon declared at a recent party meeting that the convention should adopt a clear position on extending Israeli law to the occupied West Bank and Gaza Strip, which would effectively mean the annexation of the territories.

Shamir has urged that the convention sidestep the annexation issue, because the party should not put itself on record as having advocated a violation of Israel's international commitments.

Levy, who was defeated by Shamir in a leadership struggle two years ago, has said in recent speeches that he is not seeking to undermine Shamir's position as alternate prime minister, or jeopardize the rotation. Nonetheless, a defeat of Shamir in his party would be viewed as a vote of no-confidence and would make it practically impossible for him to succeed Peres in the premiership.

Begin reportedly has indicated his support of Shamir through party channels, and today he sent a letter read to convention delegates calling for a united Herut Party leadership under Shamir's guidance -- described by observers as a pro forma move recognizing Shamir's government position.

While some Herut strategists proposed that Begin defuse the intraparty rivalries by remaining as titular head of the party, he reportedly never seriously considered a further role for himself in the party that he helped found and dominated for decades.