A security official said the attack in the city of Mukalla in Hadramout province was carried out by a suicide bomber, and that it bore the hallmarks of an al-Qaida operation. Both al-Qaida and southern separatists are active in the region. 

A health official confirmed the death toll. Both officials spoke on condition of anonymity as they were not allowed to speak to the press. 

The blast came hours after Abed Rabbo Mansour Hadi was sworn in as president to replace longtime leader Ali Abdullah Saleh, following an election aimed at ending more than a year of political turmoil in Yemen. Hadi was the only candidate in the election. 

In his televised speech before parliament, Hadi vowed to keep up Yemen's fight against al-Qaida-linked militants, who have taken advantage of the country's political turmoil to seize control of towns and swaths of territory in the country's restive south. 

Hadi also pledged to that thousands of Yemenis forced from their homes because of the fighting among government troops, southern separatists, mutinous military units, tribal movements, and numerous other factions. 

"One of the most prominent tasks is the continuation of war against al-Qaida as a religious and national duty, and to bring back displaced people to their villages and towns," Hadi said. 

Hours after Hadi spoke, the blast rocked Mukalla in the province of Hadramout, part of formerly independent south Yemen, which united with the north in 1990. 

Ahmed al-Rammah, who witnessed the blast, said by phone from Mukalla that he saw a pickup moving slowly to the gate as soldiers were coming out. Then it exploded, he said. The blast was followed by heavy gunfire from the surviving guards. 

Hadramout's governor, Khaled Said el-Deeny, told The Associated Press that police have launched an investigation. 

The province is one of many in southern Yemen that has been wracked by violence in the wake of anti-Saleh protests over the past year. Many accuse the longtime ruler of allowing security to collapse as a way of pressuring Western governments and neighboring Gulf countries into keeping him in power. 

Under international pressure late last year, Saleh signed a Gulf-brokered and U.S. backed agreement that gave him immunity from prosecution for the deaths of hundreds of people in last year's turmoil in exchange for handing over powers to Hadi, his deputy at the time. 

Hadi takes power with a popular mandate bolstered by the unexpectedly large turnout, 65%, for the Tuesday vote. 

But he faces a slew of challenges as he tries to bring stability to Yemen. He must restructure powerful security forces packed with Saleh loyalists, launch a national dialogue that would include southern secessionists, and appease a restless religious minority in the north as well as disparate opposition groups in the heartland. 

Washington has played an active role in the transition, in hopes that Hadi can head off chaos and ensure cooperation against the country's active al-Qaida branch. 

Government operations have failed to oust the group, which is blamed for trying to blow up a U.S.-bound airliner in 2009 and cargo planes bound for the US a year later. 

Saleh meanwhile returned to Yemen early Saturday after spending about three weeks in the U.S. receiving treatment for injuries he suffered during a June explosion at his compound that helped hasten his departure. 

Upon his arrival, Saleh told a local television stationed owned by one of his sons that the election was "part of a peaceful exchange of power", and that it marked the start of "a new page" in Yemen. 

Saleh is the fourth Arab leader swept from power by the Arab Spring. But thanks to his continued presence in the country and his negotiated exit, the political changes brought by his ouster may be much less dramatic than the results of uprisings in Tunisia, Egypt, and Libya. 

Many fear that the ex-president, who has cast a large web of tribal and family relations during his more than three decades of rule, may still try to pull the strings during the transitional period until a new constitution is written. 

Hadi called on all political parties to abide by democracy as a means to take Yemen out of its crisis. 

"Expected changes don't come by mere wishes and hopes but through democratic dialogue, and through a serious and correct approach to the key issues that racked the country," he said. 

The election saw several attacks against polling stations in the southern province of Aden, but Hadi vowed in his oath to preserve the country's unity. 

"I swear by Almighty God to uphold the republican system, respect and preserve the constitution and the unity and independence of Yemen," he said. 

The ceremony was attended by the US and EU ambassadors, and several Arab envoys. 

Meanwhile, senior officials close to Saleh said the former president was waiting for an answer from the Gulf sultanate of Oman on whether he can live there. Saleh stayed in Muscat in January for some days before he left to the U.S. for treatment, and Yemeni officials raised the possibility at the time that he would eventually seek exile in Oman, which borders Yemen to the east. 

The officials said Saturday that Sultan Qaboos bin Said received Saleh's request but did not meet him. They spoke on condition of anonymity because of the sensitivity of the issue, 

Saleh's son, Ahmed, also traveled to Oman on Jan. 21 to arrange a residence for his father but did not meet with the sultan at the time either. 

The officials said Oman was negotiating the issue with its Gulf Arab neighbors and the United States.