Yemeni forces backed by tanks fired on a funeral procession on Friday for a young man beaten to death in police custody, killing at least one person, a medical official said.

The burial of 25-year-old Ahmed Darwish in the southern port city of Aden turned into an anti-government protest by tens of thousands of people calling for the ouster of the country’s President. Similar protests were held around the nation, including in the capital, Sanaa.

Darwish was arrested in a mass roundup by security forces last year, before the political crisis that spun off from the other uprisings sweeping the Arab world since the start of this year. It does not appear he was involved in any political activism or with the southern secessionist movement that has simmered for years in Yemen, but his death became a rallying point for those fed up with abuses by security forces.

A forensics report published by rights groups found that Darwish was tortured to death in June of last year, and his family had refused to bury him until an investigation was concluded. A court ruling on Sunday found three policemen guilty in his killing and determined that Darwish died of beatings with metal objects, said his brother, Anwar. The policemen have not been sentenced.

As Friday’s funeral turned into an angry demonstration, government forces moved in to disperse the crowds. At least one person was killed by gunfire and six others were wounded, a medical official said, speaking on condition of anonymity because he was not authorized to brief the media.

“We are witnessing a second funeral now and a new misery that involves the killing of innocents,” said Mr. Anwar Darwish.

Yemen’s political crisis began in February with protests by largely peaceful crowds calling for President Ali Abdullah Saleh to step down after nearly 33 years in power in the impoverished country on the southern edge of Arabia. A crackdown has killed at least 167 people, according to Human Rights Watch.

The President is clinging to power despite the daily protests and an attack on his palace this month that badly wounded him and forced him to fly to Saudi Arabia for medical treatment.

In the capital on Friday, a weekly rally held in support of Mr. Saleh drew far fewer people than in recent weeks, witnesses said. By comparison, those calling for him to give up power continue to turn out en masse in an area of Sanaa they have dubbed “Change Square.”

The U.S. fears Yemen’s turmoil will give al-Qaeda’s Yemeni franchise, considered one of the network’s most active branches, more room to operate freely and plot attacks on the West from the country’s remote and mountainous hinterlands.

In Aden, a key southern port, military units have withdrawn from three main checkpoints guarding the city, leaving residents worried that al-Qaeda-linked militants who have seized control of two nearby towns could attempt a takeover of Aden.

Critics of President Saleh have accused him of allowing the Islamic militants to seize the towns as a why to bolster his argument that without him Yemen would fall into the hands of al-Qaeda.

The Army has pulled out tanks and artillery units from the entrances of Aden and police forces are also absent.

“There were three checkpoints with guards on tanks but now we see none,” said resident Adeeb Salam.

Residents in some districts have started to form popular committees to try to fill the security vacuum, according to Shaher Mohammed Said, an activist and city resident.

“We have been hearing that militants have made it through to Aden, which makes us worried about our city,” he said.

Yemen’s al-Qaeda branch has been linked to several nearly successful attacks on U.S. targets, including the plot to bomb a Detroit-bound airliner in December 2009 with a bomb sewn into the underwear of a would-be suicide attacker. The group also put sophisticated bombs into U.S.-addressed parcels that made it onto cargo flights last year.

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