GAZA CITY, Gaza Strip - Gunmen from the rival Hamas and Fatah movements battled in Gaza City for a third straight day yesterday, firing mortars and grenades in clashes that killed at six people in the increasingly bloody power struggle over the Palestinian government.

The deaths brought to at least 25 the number of Palestinians killed since late Thursday, with at least 68 people wounded and efforts to forge a coalition government at a standstill.

The latest fighting, which started after a Hamas activist was killed in a bombing, has been among the deadliest in nearly two months of clashes.

Palestinian Prime Minister Ismail Haniyeh of Hamas issued a statement calling for calm.

But in a clear jab at the Palestinian Authority president, Mahmoud Abbas of Fatah, Haniyeh criticized "troublemakers who are trying to veer away from the path of our people" by receiving "dirty American funding and arms." The White House is seeking about $85 million to help bolster Abbas' forces.

The violence has been fueled by Abbas' pledge to call early elections if talks between Hamas and Fatah fail. Abbas, who is traveling in Europe, said last week that he would move forward with a plan for new elections if the coalition talks failed to produce results within three weeks.

Hamas, which defeated Fatah in parliamentary elections last year, opposes a new vote.

Streets in the hardest-hit neighborhoods were deserted yesterday, and only bakeries and grocery stores opened for business. Gaza City's main outdoor market was closed. Al-Azhar University called off exams, and the Hamas-controlled Interior Ministry, which oversees several security forces, told its employees to go home.

Late last night, a 12-year-old boy was killed and two people were wounded in the northern Gaza Strip during a shootout between Hamas and Fatah gunmen, witnesses and hospital officials said. One of the Hamas men was killed in the gun battle, hospital officials said.

Two men were also killed in a car explosion north of Gaza City, Palestinian security officials said.

A gun battle erupted yesterday near the Islamic University, killing one man, according to hospital officials. In a firefight elsewhere in the city, a Palestinian police officer was killed.

Before dawn yesterday, Hamas gunmen fired mortars at the Abbas-allied Preventive Security Service headquarters and at the home of the force's chief, Rashid Abu Shbak, officials said.

In fighting around the compound on Friday, six Hamas gunmen were killed and a seventh died yesterday of wounds sustained in that battle, Hamas spokesman Ayman Taha said.