RAMALLAH, West Bank, AP – Hundreds of Palestinian security forces loyal to President Mahmoud Abbas went on a rampage against the Hamas-led government, riddling the parliament building and Cabinet offices with bullets before setting them ablaze in the most serious violence since Hamas won January elections.

The riots Monday cast doubt on renewed calls for Palestinian unity by leaders of Abbas’ Fatah movement and Hamas, raising new fears the Palestinians were headed toward civil war.

It also coincided with heightened violence with Israel following a blast on a Gaza beach that killed eight civilians, and prompted Hamas to call off a 16-month truce.

Palestinians say an Israeli artillery shell fired at militants’ rocket-launching operations caused Friday’s explosion. Israeli military officials said Tuesday, however, that an army investigation will conclude a Palestinian mine likely caused the blast. The military panel was expected to issue its findings later in the day.

Abbas, a moderate who was elected separately last year, has been locked in a bitter power struggle with Hamas, which does not recognize Israel and has refused to disarm its militia, despite punishing cutoffs of international funding. The dispute, which has spilled over into militias loyal to both sides, has focused largely on control of the powerful security forces.

Twenty Palestinians have been killed in infighting over the past month — mostly in Gaza, Hamas’ stronghold. Monday’s violence signaled that Fatah is now ready to move the conflict to its West Bank power center.

Late Monday, hundreds of members of the Fatah-dominated Preventive Security force shot out the windows of the parliament building before storming the two-building Cabinet complex, where they smashed furniture, destroyed computers and tore up documents. No casualties were reported.

Shooting wildly in the air, the mob then set fire to one of the Cabinet buildings, gutting the fourth floor. When a fire engine approached the scene, one gunman lay on the road, preventing it from reaching the building.

“Every time they touch one of ours in Gaza, we will get 10 of theirs in the West Bank,” said one member of the security force. Dozens of gunmen from a pro-Fatah militia joined the mob.

The crowd also set fire to the parliament building and a Hamas office. Both blazes were quickly contained. Abbas’ presidential guard later arrived to guard the burnt-out buildings.

Late Monday, Fatah gunmen briefly abducted a Hamas lawmaker, Khalil Rabei, after attacking his office and setting it on fire. Rabei said he was kicked and threatened before he was released.

Abbas’ personal guard granted refuge to Rabei and nine other Hamas lawmakers, and was ordered to escort the men home safely.

The rampage in Ramallah followed an attack by Hamas gunmen on a Preventive Security installation in Gaza, which set off daylong clashes that left two people dead and 14 wounded.

Abbas was in Gaza at the time of the rampage, where he has been holding negotiations with Hamas in hopes of ending the political deadlock.

“The president strongly condemns the attacks and the assault against the public institutions and the building that took place in the West Bank tonight,” said a statement issued by Abbas’ office.

Early Tuesday, several thousand Hamas supporters demonstrated outside the Palestinian parliament building in Gaza City. They condemned the violence in Ramallah, and threatened retaliation against the Preventive Security forces.

“What happened today in Ramallah is very shameful and I think the president did not use all his efforts to prevent it,” said Hamas spokesman Sami Abu Zuhri.

Hamas planned another demonstration in Ramallah later Tuesday, and Fatah planned a protest in the West Bank town of Jenin.

Officials said Abbas had appointed a retired commander to serve as a military adviser and liaison to the Hamas-run Interior Ministry. The appointment of Lt. Gen. Abdel Razek Majaide, who is respected by Fatah and Hamas alike, could help to ease tensions.

Internal tensions have spiked since Abbas scheduled a July 26 referendum on a plan that calls for recognition of Israel. Abbas has endorsed the plan as a way out of the stalemate, but Hamas objects to the vote.

In a conciliatory gesture on Monday, the Hamas-dominated Palestinian parliament delayed until June 20 a vote on a plan to block the referendum. But the violence cast serious doubts on the efforts to bridge the gaps.

While Abbas was trying to persuade Palestinians to rally behind his peace efforts, Israeli Prime Minister Ehud Olmert was in Europe trying to sell his planned West Bank pullback.

In London on Monday, Olmert said he would make “every possible effort” to negotiate peace with the Palestinians but will act unilaterally if no agreement can be reached.

Under Olmert’s program, Israel would pull out of most of the territory and dismantle dozens of Jewish settlements — but also retain key areas, including land around Jerusalem and part of the city the Palestinians seek for their capital.

Olmert said the pullout would encompass 90 percent of the West Bank. And he left the door open to future negotiated pullouts even if the initial phase is unilateral and partial.

“What will happen when we will come to the point of the 90 percent of the territories, and the remaining 10 percent remains to be seen?” he said. If “the day will come when the Palestinians will be ready to negotiate with us … I will then be willing to come to discuss it in detail.”

Israel, which withdrew unilaterally from all of the Gaza Strip last year, has repeatedly said it would not negotiate with the Palestinians unless Hamas first disarms its militia, recognizes Israel, and accepts past peace agreements.

Olmert is to meet with officials in Paris on Tuesday.