GAZA CITY, Gaza Strip ? Members of a new Hamas security force and Palestinian police traded gunfire in the streets near the parliament building in downtown Gaza City on Monday, killing an aide to the Jordanian ambassador in Gaza and wounding at least 10 people.

The running battles, which sent people ducking into doorways as gunmen fired from behind buildings and from rooftops, raised fears of civil war fueled by a power struggle in the Palestinian Authority.

The Hamas-run Interior Ministry posted a 3,000-strong security force on the streets last week in defiance of Palestinian President Mahmoud Abbas, heightening tensions with forces loyal to Abbas and dominated by members of his Fatah party.

The clash, the worst since the new force was deployed, underlined the challenges facing Abbas, who is struggling to assert his leadership of the Palestinian Authority after Hamas trounced Fatah in parliamentary elections in January.

The Hamas-led Palestinian government has resisted Abbas’ calls to renounce violence and accept previous agreements with Israel, while tangling with him over control of the security forces.

There were conflicting reports of what set off Monday’s clash. According to some accounts, members of the Hamas force fired at a vehicle carrying officers from the Preventive Security Service, a branch loyal to Abbas, suspecting that its occupants were about to shoot. Hamas said its members came under fire first.

An armed Palestinian member of the new Hamas security force, right, patrols a street just in front of a group of Palestinian policemen in the outskirts of the Khan Younis refugee camp in the southern Gaza Strip. Hamas militiamen and Palestinian police traded heavy fire near Gaza City's parliament building on Monday, killing an aide to the Jordanian ambassador in Gaza and wounding 11 people in the worst internal fighting in recent weeks.

Khaled Abu Hilal, a spokesman for the Interior Ministry, said that a vehicle carrying gunmen pulled up and fired at both sides, causing them to shoot at each other.

The gunfight spread through the streets, with ordinary police officers joining the fray. Gunmen took cover behind trees, cars, and walls, as pedestrians fled for cover, including a group of black-clad women who had been marching with signs to protest the armed chaos.

Khaled Radaida, 55, an aide to the Jordanian ambassador, was killed when his car was caught in the crossfire.

Jordan demanded an immediate inquiry, and Palestinian Prime Minister Ismail Haniyeh called the Jordanian ambassador, offering condolences and promising an investigation. The incident came amid increased tension between Jordan and Hamas, after Jordan recently accused the group of smuggling weapons into the kingdom for use in attacks against officials and public institutions.