Hamas gunmen and Palestinian security forces clashed Sunday evening in the Gaza Strip.

A police officer and two civilian bystanders were killed in the hours-long gun battles, and at least 50 others were wounded in the most sustained factional fighting since Israel withdrew from Gaza three weeks ago.

Palestinian authorities and Hamas officials gave conflicting accounts of how the clashes began. Hamas officials said gunfire broke out after Palestinian police sought to disarm a group of Hamas fighters riding in a taxi in Gaza City. Among those in the car were Mohammed Rantisi, son of Abdel Aziz Rantisi, the Hamas leader killed last year in an Israeli airstrike.

The Israeli government and the Bush administration have urged the Palestinian Authority to establish firm control over Gaza, now the testing ground for a future Palestinian state, at a time when Hamas is making a strong play for power.

Palestinian officials said the violence began with a dispute at an automated teller machine involving a Hamas member. When Palestinian police arrived to settle the matter, Hamas gunmen tossed grenades at the men, authorities said. Witnesses said Hamas gunmen later used RPG-7 grenade launchers to attack two police stations.

The clashes came after Israeli officials announced the suspension of large-scale military operations in Gaza that began Sept. 24 after members of Hamas fired a barrage of rockets into southern Israel. The group, formally known as the Islamic Resistance Movement, accused Israel of igniting an explosion at a Hamas rally in the Jabalya refugee camp a day earlier that killed 17 people. Israeli military officials denied any involvement in the explosion, and Palestinian security officials said it was probably an accident.

Over the course of the operations, Israeli military aircraft targeted roads, buildings and Hamas militants in Israel's first military response to attacks from Gaza since its withdrawal. The military also used artillery for the first time, firing into open border areas used to launch rockets. In the West Bank, Israeli soldiers arrested 441 Palestinians, many of them members of Hamas.

"We have suspended certain operations pending the actions of Palestinian security forces, specifically to dismantle terrorist infrastructures and disarm Hamas as a first step," said Raanan Gissin, spokesman for Israeli Prime Minister Ariel Sharon. "We'd like to see if the Palestinian Authority is willing and capable of taking advantage of the new situation. Now is the opportunity for them to take control."

Sharon has demanded that the Palestinian Authority, the provisional Palestinian government in Gaza and the West Bank, crack down on Hamas and other militant groups at war with Israel. Palestinian leader Mahmoud Abbas reached an agreement with Hamas and other factions last week to end armed public demonstrations of any kind after the Jabalya explosion.

The two groups are bitter political rivals. Hamas, which does not recognize Israel's right to exist, has emerged as the leading Palestinian opposition party. But Fatah, which dominates the Palestinian Authority, improved its showing last week in a round of municipal elections, the first since Israel left Gaza.

The parties are positioning themselves for parliamentary elections scheduled for January, the first that Hamas plans to contest. Sharon has threatened to undermine those elections if Hamas is allowed to compete without disarming first.

Palestinian and Israeli officials said Sharon and Abbas would meet soon. The two were initially scheduled to meet Sunday, but the summit was postponed last week because of the Gaza military operation.

Sharon aides said the meeting -- the first since the Gaza withdrawal -- would probably take place before Abbas's scheduled Oct. 20 trip to Washington and include discussion of the Gaza security situation and the January elections.

Members of the al-Aqsa Martyrs Brigades rally in the Balata refugee camp in the West Bank. Israel has demanded a crackdown on militia groups.