-- Gunmen stormed into a mourning tent for Yasser Arafat in Gaza City on Sunday night shortly after the arrival of the new leader of the Palestine Liberation Organization, firing weapons and setting off a chaotic clash with Palestinian police in which at least two officers were killed, according to witnesses and medical officials.

Bodyguards for Mahmoud Abbas, a former prime minister, shoved him to the ground, and hundreds of other people dived for cover as the gunmen and security forces traded fire, according to witnesses and television footage.

As head of the PLO, Abbas now is regarded as the most powerful Palestinian leader following Arafat's death.

Abbas said later that the clash was not an assassination attempt. Speaking on Palestinian television, he said that "because of the crowdedness at the location there was anarchy and there was shooting in the air, and therefore we requested the people to leave."

Yasser Abed Rabbo, a Palestinian government spokesman, said it was a "spontaneous" clash between armed groups, not a planned attack.

The incident -- coming just three days after Arafat's death and involving one of the four men who have assumed his responsibilities -- underscored widespread Palestinian concerns about violence and lawlessness that could erupt if a power vacuum takes hold. Arafat, who was 75 at his death, groomed no successor.

The Palestinian Authority, which Arafat headed, on Sunday announced that elections for its president would take place Jan. 9.

About five minutes after Abbas arrived at the mourning tent in Gaza City, approximately 20 armed men burst into the tent where he and other senior Palestinian leaders, including former Gaza security chief Mohammed Dahlan, were greeting local officials and residents paying their respects to Arafat, who was buried in the West Bank city of Ramallah on Friday.

"Abbas and Dahlan are agents for the Americans!" the gunmen shouted, according to footage taken by Associated Press Television News. Some of the gunmen yelled, "No to Abbas! No to Dahlan!"

The gunmen began firing into the air and uniformed Palestinian security guards returned fire as Abbas's bodyguards pushed him to the ground. Screams filled the air, with hundreds of people seeking cover and overturning white plastic chairs.

Witnesses said the gunmen were from the al-Aqsa Martyrs Brigades, the armed wing of the mainstream Fatah political movement, of which Abbas is now second in command, the Arab satellite network al-Jazeera and other news outlets reported.

The al-Aqsa Martyrs Brigades, one of the main militant groups that has sponsored suicide bombings against Israelis, is highly decentralized, and its militant cells often act on their own. It was unclear whether the incident signaled a significant backlash by the group against Abbas, a moderate.

A Fatah leadership council in Ramallah had earlier announced, apparently in error, that Abbas would be the movement's candidate. But Abbas told al-Jazeera that the announcement was "premature," and that he had not yet been selected as the group's candidate.

Such a move by Fatah would preempt the candidacy of anyone else, particularly Marwan Barghouti, the former head of Fatah in the West Bank and a reputed founder of the al-Aqsa Martyrs Brigades who is serving a life sentence in an Israeli prison.

It could not be determined whether the gunmen in Gaza were angry about the announcement that Fatah had selected Abbas over Barghouti. If that were the case it would signal a much-anticipated battle between Fatah's old guard and young reformers who have done most of the fighting in the current Palestinian uprising.

Gaza has been a scene of chaos in recent months. Several top security officials and foreigners were kidnapped in July by Palestinian militants to publicize their demand for reforms and to protest corruption among officials in the governing Palestinian Authority.

Militant groups that have sponsored suicide bombings against Israelis -- particularly the Islamic Resistance Movement, known as Hamas, and Islamic Jihad -- have become extraordinarily popular in Gaza during the Palestinians' four-year uprising. They are well armed and financed, and their power rivals that of the official security forces under the Palestinian Authority.

Both Palestinian and Israeli officials have expressed concern that the security situation in Gaza could deteriorate significantly, perhaps even setting off a Palestinian civil war, if Israel goes ahead with Prime Minister Ariel Sharon's plan to withdraw Israeli troops and Jewish settlers from the Gaza Strip early next year and Palestinian security forces do not assert their authority. Politicians and security officials on both sides fear that a bloody power struggle could develop in such a security gap if rival groups fight for control of the streets.

Mahmoud Abbas, a former Palestinian prime minister, is led out of a tent erected for mourners of Yasser Arafat in Gaza City after shots were fired.