A summit meeting of about half the leaders of the Arab world convened here today amid persistent confusion and declining expectations about what it might achieve.

Morocco's King Hassan II, who convened the conference as chairman of the Arab League, called for conciliation with those nations that did not attend.

But Yasser Arafat, chairman of the Palestine Liberation Organization, took the floor in what appeared to be an unexpected power play and insisted on denouncing Syrian-backed attacks against Palestinians in Lebanon.

Syria, Algeria, Libya and Lebanon were the key nations that refused, along with South Yemen, to answer Hassan's call to this emergency session. Nonetheless, this is the first Arab summit successfully convened since 1982.

To bring it about, the Arab League dispensed tonight with the principle of unanimity, which has governed its decisions in the past and which several Arab leaders have said held all decisions hostage to the consent of the extremes.

Hassan declared at the outset that decisions would not and could not be made by consensus because some of the 21 members currently in good standing were absent.

In what is likely to be the central thrust of the conference, he said, "I hope to God that we produce results that contribute to the next summit and contribute to bringing all brothers together."

Arafat clearly was not content to see such a tone taken at this emergency meeting originally sought after attacks on Palestinian refugee camps in Lebanon by Syrian-backed Shiite Moslem Amal militiamen.

The Palestinians have said they hope this meeting will challenge Syria's claim to Arab leadership.

Arafat, in his accustomed dramatic style, likened the convening of the summit to the action of a Samaritan answering the cries of a woman being raped. In the context, there was no question that he considered Syria the rapist.

Arafat also called for measures to end the Iranian-Iraqi war and efforts to put the Arab cause high on the list of subjects to be discussed in the coming talks between President Reagan and Soviet leader Mikhail Gorbachev.

The basic lineup at the conference seems likely to run against Arafat's wishes. The pivotal role almost certainly will be played by Saudi Arabia, represented by Crown Prince Abdullah.

Abdullah has longstanding ties to the Syrians, and many Arab commentators, including well-informed Saudis speaking privately, have suggested that Abdullah is likely to represent Syria's interests here.

If that proves to be the case, little can be expected beyond general reaffirmation of past Arab League positions and proposals for how to make the next regular summit work if it eventually convenes.

Until late today it was not clear which countries finally would attend. Libya sent a low-ranking delegation to consultations earlier this week but none to the summit itself.

In addition to Hassan and Arafat, who is considered a head of state in the Arab League, others attending are the leaders of Bahrain, Jordan, Somalia, Djibouti, Sudan, Mauritania and Qatar. High-ranking delegations from six other states, including Iraq, also are attending.