An explosion on Wednesday ripped through a Yemeni military vehicle in the south, killing five soldiers, while another four civilians died in the ensuing firefight.

The blast hit the vehicle close to a busy market selling qat, the mildly stimulating leaf that Yemenis addictively chew in a province known as a stronghold for the local branch of the al-Qaida militant group.

"We heard the sound of explosions and saw fire coming out of the car. The soldiers were lying on the ground," said eyewitness Ali Dahmash.

The panicked surviving soldiers began firing in the air, and were joined by soldiers rushing to the scene. Four civilians in the marketplace were killed by the gunfire, and more than 20 were wounded said Dahmash.

The explosion occurred in the southern Yemeni town of Zinjibar, in a province known as a stronghold for Muslim militants, including al-Qaida.

Yemen is home to one of the most active branches al-Qaida, but military officials, speaking on condition of anonymity because they were not authorized to speak to the press, said they weren't sure who the assailants were.

Extremists regularly attack soldiers in the lawless, impoverished country, which is also wracked by pro-democracy demonstrators calling for the ouster of their longtime authoritarian ruler, Ali Abdullah Saleh.

On Wednesday, tens of thousands gathered in Yemen's chief cities, as they have for the past months to reiterate their demands that Saleh step down after more than 30 years in power.

More than 140 people have died since the protests began nearly three months ago.

In the southern province of Aden, a general strike shuttered shops, closed schools and halted commercial transport, in the latest efforts to pressure Saleh to leave.

This week, Saleh spurned mediation efforts by the powerful neighboring bloc of Gulf countries that would have had him step down in return for political immunity.