NABLUS, West Bank — Thousands of angry activists from the defeated Fatah Party demanded their leaders' resignations, Palestinian police stormed a parliament building in Gaza and other security forces clashed with Hamas gunmen — signs of growing instability following Hamas' victory in parliamentary election.

Fatah gunmen climbed on top of the Palestinian parliament building in Ramallah, fired in the air and posted a picture of the late leader Yasser Arafat on the roof to cheers and whistles from hundreds of supporters below. Dozens of armed police officers briefly stormed the building in Gaza City and demanded an immediate trial for Hamas members who killed police in fighting in recent months.

"Everybody should know that we are not going to allow the Interior Ministry to belong to Hamas," the police said, referring to the government body that controls the security forces.

Most of the 58,000 members of the security forces are allied with Fatah and fear for their jobs under a Hamas-led government. Hamas has its own armed force of about 5,000 gunmen in Gaza.

In earlier fighting in Gaza, Hamas gunmen wounded two Palestinian policemen in what authorities said was a roadside ambush early Saturday, hours after two officers and a Hamas activist were wounded in another firefight.

Ismail Haniyeh, a Hamas leader in Gaza, addressed the growing fears in a mosque sermon Friday, saying there would be no purge of the security forces. However, it appears inevitable that Hamas will want to replace many of the officers with their own loyalists.

Wednesday's election exposed deep tensions within Palestinian society and was a clear rebuke to Fatah for its corruption and inability to maintain order. Before the vote, veteran Fatah leaders, those most tainted by corruption allegations, resisted repeated calls for reform by the Fatah young guard.

In Damascus, Hamas' top leader, Khaled Mashaal, reiterated Saturday that his group seeks a partnership with all political parties, but also wants to reform the government. In a reference to Fatah, Mashaal warned that those "who might try to block the work because they are out of power" would be held responsible if reforms are blocked.

In Fatah, there was growing finger-pointing following the defeat, which ended four decades of Fatah dominance in Palestinian politics. Demonstrators demanded the resignation of the party's entire central committee. Mahmoud Abbas, who is part of the committee, was elected last year as Palestinian Authority president. Only a few Fatah activists called for him to step down.

About 2,000 Fatah members marched in the West Bank city of Nablus, led by dozens of gunmen from the Fatah-allied Al Aqsa Martyrs' Brigades, who fired in the air from the back of a truck.

"We are now no longer part of the cease-fire," one of the gunmen, Nasser Haras, told the crowd. Palestinian militants groups agreed last year to a cease-fire with Israel.

The protests began Friday, soon after Abbas said he would ask Hamas to form the next government.

Hamas' Haniyeh said he asked Abbas to meet Sunday to discuss forming a government, but Abbas' office said no appointment had been made. Hamas, which has no experience in governing, took 76 of the 132 parliament seats up for grabs.

Ghazi Hamad, one of Hamas' ideologues, said the group might form a government of technocrats with no connection to Hamas. Such a government might relieve some of the international pressure on the group.

Israel, caught off guard by the Hamas parliamentary landslide after its vaunted intelligence services predicted a slim Fatah victory, said it would have no contacts with a Palestinian government that includes Hamas.

Acting Foreign Minister Tzipi Livni appealed to the international community not to legitimize a Hamas government, saying elections "are not a whitewash for terror."

Hamas, responsible for dozens of suicide bombings on Israelis, has long called for the destruction of the Jewish state. In recent years, some Hamas leaders grudgingly accepted the idea of a Palestinian state in the West Bank and Gaza, but only as a stage toward freeing the rest of Palestine — meaning Israel.

President Bush said in an interview with "CBS Evening News" that the United States would cut aid to the Palestinian government unless Hamas abolishes the militant arm of its party and stops calling for the destruction of Israel.

Hamas is listed as a terror organization by the United States and the European Union.

Jacob Walles, the U.S. consul-general in Jerusalem, said the United States gives $400 million a year to the Palestinian Authority.