SANAA, Yemen -- A large explosion shook the southern Yemeni port city of Aden on Friday, killing three security personnel, as anti-government demonstrators there and in other cities demanded the ouster of the country's autocratic leader of nearly 33 years.
The blast, in Aden's free-trade zone, was heard throughout the city, but its cause was unknown. It blew out the glass facade of a four-story building. Besides the three killed, three others were injured, said a medical and a security official who spoke on condition of anonymity because they were not authorized to talk to journalists.
Residents of Aden say military forces loyal to embattled President Ali Abdullah Saleh recently removed checkpoints at the city's entrances and withdrew, raising fears that Islamic militants who seized two nearby towns after government forces carried out a similar pullback could attempt a takeover of the strategic port city.
Months of political turmoil in Yemen have raised fears, perhaps most acutely in the U.S., that Yemen's Al Qaeda franchise could end up with more room to operate freely and plot attacks on the West from its redoubts in the country's remote and mountainous hinterlands.
The U.S. says Al Qaeda in the Arabian Peninsula is now the terror network's most active branch. It 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 but were caught before they exploded.
Yemen's political crisis began in February with protests by largely peaceful crowds calling for Saleh to end his rule over the impoverished country on the southern edge of Arabia. A government crackdown on unarmed protesters 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.
Over the past weeks, Islamic militants, some of whom are Al Qaeda-linked, have taken advantage of the strife to overrun the southern towns of Jaar and Zinjibar, which are not far from Aden. This week, nearly 60 Al Qaeda suspects broke out of a prison in Yemen.
On Friday, residents of Aden spoke of worries that militants could reach their city after the sudden disappearance of tank and artillery units from the entrances of the city. Police are also absent, they say.
In the earlier takeovers of Jaar and Zinjibar, smaller towns to the east of Aden, security forces also disappeared, raising accusations that President Saleh allowed the militant takeover to bolster his claims that without him in power, Al Qaeda would seize control of the country.
Saleh's opponents have dismissed his warnings as overblown. Al Qaeda in the Arabian Peninsula has an estimated 300 hard-core members, and is not seen as capable of seizing control on a wide scale.
The United States, with the agreement of Saleh loyalists, has carried out expanded strikes against Al Qaeda targets in Yemen with armed drones and warplanes in recent weeks.
Adeeb Salam, a resident of Aden, said the city's entrances are not protected. "There were three checkpoints with guards on tanks, but now we see none," he said.
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.
The only visible security forces in Aden on Friday were those confronting tens of thousand of anti-government protesters who took the streets during a funeral procession for a young man beaten to death in police custody.
Forces backed by tanks fired to disperse the crowds, killing at least one person, a medical official said, speaking on condition of anonymity because he was not authorized to brief the media.
The man who was buried, 25-year-old Ahmed Darwish, was arrested in a mass roundup by security forces last year, before the start of this year's political crisis, which spun off from the other uprisings sweeping the Arab world.
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.
To the north of the capital on Friday, Republican Guard forces fired missiles at the town of Arhab, killing one resident and injuring 13 others, said resident Mohammed Ghaylan. Three homes were destroyed, he said.
Residents there have supported the anti-government protesters.