The sudden show of force came after two Hamas members were gunned down in drive-by shootings hours earlier that the group blamed on Abbas' Fatah Party. There were no reported clashes with Fatah loyalists, but the Hamas force did violently break up a peaceful protest by prospective teachers in one southern town.
The deployment defied a presidential order and added to tensions that have been rising since Hamas defeated Fatah in legislative elections in January. The power struggle already has spilled over into violence, and the Palestinian territories increasingly appear headed toward a bloody showdown.
The bulk of the new Hamas force was sent to chaotic Gaza City, where bearded gunmen in black T-shirts and green vests took up positions along the main streets and at busy intersections, near banks and outside ministry buildings and parliament. In one display, dozens of masked gunmen marched in formation through a major square.
Outside the Bureij refugee camp in central Gaza, members of the new force stood a few feet away from members of a security branch that answers to Abbas.
And in the southern city of Khan Younis, about 40 members of the new force pulled up to the Education Ministry, jumped from their jeeps and fired in the air to break up a peaceful protest of recent college graduates who want teaching jobs. The teachers were protesting an application fee.
The gunmen moved into the building, where they bludgeoned protesters with clubs and rifles, demonstrators said.
"We were protesting peacefully, and suddenly these gunmen came and assaulted us," said a protester as he applied a bandage to a small gash on his head. "We don't know who they are or why they came here." He identified himself only as Khaled, saying he feared retribution.
The new Hamas force is headed by Jamal Abu Samhadana, a bombmaker wanted by Israel who is suspected of masterminding a deadly attack on a U.S. diplomatic convoy in 2003.
Hamas officials said the new force's aim was to bring order to Gaza, where marauding gangs of armed men routinely terrorize citizens. The deployment was ordered after drive-by shootings in Gaza killed two Hamas militants.
There was no claim of responsibility for the drive-by shootings, which came after cars belonging to senior Fatah militants were blown up in Gaza City. But Hamas spokesman Mushir al-Masri blamed Preventive Security, a force filled with Fatah supporters.
"This is a plan of the Preventive Security to draw Hamas into a civil war and to make the government appear ineffective," al-Masri said.
The dispute between Abbas, a moderate who was elected separately a year ago, and Hamas has focused on control of the security forces.
Abbas infuriated Hamas last month when he appointed a loyalist to head three security agencies that report to the Hamas-controlled Interior Ministry. Hamas responded by announcing it would create a militant force of its own — even after Abbas vetoed the plan.
Tayeb Abdel Rahim, a presidential aide, urged Hamas to reverse the decision. "This force doesn't serve security. On the contrary, it creates problems," he said Wednesday.
The new force drew mixed reactions from the public, reflecting the deep rift among the 1.3 million residents of Gaza, an impoverished, densely populated coastal strip.
In the town of Deir el-Balah, Hamas supporter Sufian Abu Mustafa welcomed the new force and offered the men cold water. "It's a very good step. We need even more than 3,000 to end the chaos," Abu Mustafa said.
But Abdel Aziz Shaheen, a retired Fatah lawmaker, held a solitary demonstration against the force outside the Palestinian parliament building in Gaza City.
"I don't want to be ruled and governed by militias," he said. "Maybe I am alone today, but I am sure that in the future we are going to see thousands of people protesting."
Also Wednesday, two Islamic Jihad militants were killed and another was wounded in a gunbattle with Israeli soldiers in the West Bank city of Nablus. Islamic Jihad has been responsible for all nine suicide bombings against Israeli targets since Palestinian factions agreed to an informal truce early last year.
Israel said a captured former Palestinian finance official has admitted using foreign donor money to finance millions of dollars in arms purchases on direct orders from the late Palestinian leader Yasser Arafat.
Israeli Prime Minister Ehud Olmert's office said Fouad Shobaki, seized by Israel in a March raid on a Palestinian prison, told interrogators that as head of finance for the Palestinian security services, he channeled at least $9 million in aid money and public funds to buy weapons.