RAMALLAH, West Bank – Hundreds of Palestinian security men loyal to President Abbas went on a rampage against the Hamas-led government yesterday, riddling the Parliament building and Cabinet offices with bullets before setting them ablaze in retaliation for an attack by Hamas gunmen in the Gaza Strip.

The violence was the most serious in the West Bank since Hamas defeated Mr. Abbas’s Fatah movement in legislative elections in January and raised new fears the Palestinians could be headed toward civil war. It also cast doubt on renewed calls for Palestinian unity by the rival factions.

Mr. Abbas, a moderate who was elected separately last year, has been locked in a bitter power struggle with Hamas. The dispute has focused largely on control of the powerful security forces.

In yesterday’s unrest, hundreds of members of the 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,” a member of the Preventive Security force said. Dozens of gunmen from the Al Aqsa Martyrs’ Brigades, 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. Mr. Abbas’s presidential guard later arrived to guard the burnt-out parliament and Cabinet buildings.

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

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

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

Most of the Palestinian infighting in recent weeks has occurred in Gaza, Hamas’s stronghold. Yesterday’s incident signaled that Fatah is now ready to move the conflict to its West Bank power center.