Abu Nidal, the renegade Palestinian who has spent a decade fighting a secret war of terrorism against Palestine Liberation Organization Chairman Yasser Arafat, has emerged from the underground to make common cause with rebels challenging the PLO leader's authority.

Apparently buoyed by the mutineers' defiance of Arafat and presumably encouraged by his own Syrian government sponsors, Abu Nidal and his followers have attacked Arafat openly as a "traitor" and hinted that he, like other Palestinian traitors they brag of having executed, may soon be condemned by the Palestinian people to a like fate.

So confident has Abu Nidal become as a result of the mutiny that, for the first time since he was expelled from Arafat's Fatah organization in 1973 and went underground, he has eased the secrecy of his own rival organization, called the Fatah Revolutionary Command, and tentatively begun public operations in Damascus, for several years one of his principal secret bases of operation.

Abu Nidal, whose real name is Sabra Banna, has set up an information office for his organization on a quiet side street near the diplomatic quarter of the Syrian capital.

From there his followers--who until now largely had reputations as trained assassins--have begun to propagandize for their movement, openly seek recruits among young Palestinians in the city's overflowing refugee camps and even meet with the odd foreign journalist who stumbles across their path.

Abu Nidal's message that Arafat is a traitor to the Palestinian cause because of his moderation has not changed since he was drummed out of Fatah by Arafat--and condemned to death in absentia--for refusing to bow to the organization's policies of abandoning terrorism as a prelude to exploring diplomatic solutions to the Palestinian issue.

What is new is that after a decade of waging a war of assassination against Arafat supporters, variously with the backing of the Iraqi and Syrian governments, Abu Nidal feels that Arafat is in deep enough trouble among his followers that he now can openly proclaim his message from fixed offices in Damascus without fear of retribution from the mainline Palestinian organization that Arafat still heads.

More importantly, the anti-Arafat mutiny led by Abu Musa in Lebanon's Bekaa Valley has suddenly made Abu Nidal's vendetta seem less heretical than it had been considered in the past among Palestinians.

That Abu Nidal's public operation is still tentative is clear from the heavy steel shutters that cover the streetside windows of his information office, the video monitors that look up and down the street, the double steel doors, the peepholes and the automatic cameras that scrutinize each visitor.

Inside the Fatah Revolutionary Command offices, the organization's politics are clearly exposed by the grim photographs of the movement's latest martyrs--five young men executed on Arafat's orders in the Bekaa in May because they allegedly had plotted to kill two of his most trusted deputies, Salah Khalif and Khalil Wazir.

It can be seen from a slick anti-Arafat poster on the walls that Abu Nidal's group, which touched off last summer's Israeli invasion of Lebanon by shooting Shlomo Argov, Israel's ambassador to London, has become more sophisticated in presenting its message. The poster superimposes pictures of Palestinians massacred last year in Beirut's Shatila refugee camp with those of Arafat mounted like an Arab notable on horseback, at his new headquarters in Tunisia.

Several days later after my visit to the Damascus office, I interviewed a young Palestinian militant follower of Abu Nidal in a luxury hotel room in the Syrian capital. He was clearly trained by his leader to talk to a foreign audience with words rather than joining others from the group as they seek to implement their policies with guns.

"We extend our hand to all true revolutionaries fighting for Palestine and our Arab redemption against the forces of reaction, imperialism and Zionism," said the spokesman, who gave his name as Ali Saber. "In that regard we are with Abu Musa, though with nuances."

Saber openly admitted that his organization had sought to assassinate all those in the Palestinian movement whom, he said, "the Palestinian people condemned" for backsliding into "treason."

This was the case, Saber says, for Issam Sartawi, the moderate Arafat aide assassinated in Portugal last April for his talks with Israeli liberals, as well as for half a dozen other Arafat and PLO officials killed since 1978 in European and Arab capitals as a result of Abu Nidal's feud with Arafat.

While refusing to say directly whether Arafat was also a target for the movement's assassins, Saber left little doubt in his references to the PLO chairman in other questions that this was the case.

"We will continue our activities against all those, like Sartawi, who are traitors," Saber said. "It is the Palestinian people who condemn them; we execute them."

"Arafat himself has taken the road of treason," he said. "He has taken the road away from revolution, away from armed struggle, away from the statutes of Fatah and there comes a day when reality becomes very clear before the Palestinian people."

Saber accused Arafat of having become a "reactionary" in the hands of such "reactionary" Arab leaders as the Saudi princes, King Hussein of Jordan and King Hassan of Morocco. Saber said that his movement had neither a Marxist nor Arab Socialist (Baathist) ideology and believed only in revolution to redeem the Palestinian people dispossessed of their lands by Israel.

Arafat, Saber said, was already "politically finished" and had no future role in the Palestine revolution. The question of whether he was to be marked for execution, he said, "is up to the Palestinian people to decide."

Saber indicated that Abu Nidal, who has moved between Baghdad and Damascus over the years, was in neither capital that day. "It is not important whether he is in Iraq, Syria or Algeria," he said. "We are everywhere in the Arab world and elsewhere and that is what matters."

Western intelligence sources in Damascus believe Abu Nidal's small organization has three other offices in Damascus and maintains a training camp for its men in the Syrian-controlled Bekaa Valley.

Western diplomats here see the tentative emergence of Abu Nidal and his followers from hiding as another sign of the Syrian government hand behind the mutiny against Arafat's leadership of the PLO. Abu Nidal has long been considered to be very closely allied to the Syrian government and could not operate from Damascus without its approval.