Gunmen have attacked the headquarters of Yemen's domestic intelligence agency in the city of Aden, killing at least 10 security officers, officials say.
There were heavy exchanges of gunfire and plumes of smoke could be seen rising from the building afterwards.
The attackers escaped with several suspected militants detained there.
Local officials blamed the raid on al-Qaeda, which has urged supporters to take up arms against the government in response to a crackdown in the east.
The assailants - dressed in military uniforms - pulled up outside the heavily guarded compound, which is situated in the al-Tawahi district near the city's port, in two cars at around 0740 local time (0440 GMT), officials said.
They then opened fire with rocket-propelled grenades and machine guns, before storming the central courtyard and setting free several suspected militants, the officials added.
The attack came amid a flag ceremony at the Political Security Organisation (PSO) headquarters, one security official told Reuters news agency
"The high number of casualties was due to the fact that the attack came during the morning flag salute," the official said.
Witnesses told the AFP news agency that the assailants were later "seen leaving the building in a bus, taking people who had been detained there with them".
Medics told the agency that three female cleaners had also died during the raid.
In 2003, 10 men escaped from the same building, including one later convicted of involvement in the plot to blow up the USS Cole in Aden's harbour in 2000.
Saturday's attack caused a fire at the compound, which officials said was being brought under control. The military has sent troops to the area to secure it.
Yemen's government is facing three different threats - from al-Qaeda, southern secessionists, and a rebel movement in the north - although it has it has called a truce with the latter.
Al-Qaeda in the Arabian Peninsula (AQAP), formed in 2009 by a merger between two regional offshoots of the Islamist militant network in Yemen and neighbouring Saudi Arabia, has taken advantage of the instability and established strongholds in the country's largely autonomous tribal regions.
It has claimed to have been behind a number of attacks in the two countries over the past year, and has been blamed for attempting to blow up a US passenger jet as it flew into Detroit on Christmas Day.
Earlier this week, the group called on tribes in eastern Yemen to help it "light up the ground with fire under the tyrants of infidelity in the regime" in retaliation for alleged air strikes in the region.