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Yemen's newly elected president Abd-Rabbu Mansour Hadi took the constitutional oath to become Yemen's new president on Saturday. (Khaled Abdullah/Reuters)

A car bomb outside the gate of a presidential compound in a southern Yemeni city killed at least 25 people hours after the country's new president was formally inaugurated and vowed to fight al-Qaeda.

A security official said it was a suicide blast, and that it bore the hallmarks of an operation by the militant group. 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 as Abed Rabbo Mansour Hadi was sworn in as president to replace longtime leader Ali Abdullah Saleh, following an election aimed at ending over a year of political turmoil. Hadi was the only candidate in the election.

In his televised speech before parliament, Hadi swore to keep up Yemen's fight against al-Qaeda-linked militants, who took advantage of the country's upheaval to seize control of several parts of the country.

Hadi also pledged to work to bring home the thousands of internal refugees created by fighting between government troops, southern separatists, mutinous military units, tribal movements, and numerous other factions.

Hours later, the bomb exploded in the city of Mukalla in the province of Hadramout, part of formerly independent south Yemen, that joined 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.

Fear of Saleh loyalists

Newly inaugurated President Hadi has an onerous task ahead of him bringing 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.

An unexpectedly large turnout for the Tuesday vote gives Hadi a strong popular mandate to tackle these problems.

The election was arranged as part of a U.S- and Gulf-backed power transfer deal signed in November. Washington has played an active role in the transition, in hopes that Hadi can head off chaos and ensure co-operation against the country's active al-Qaeda branch.

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 on his compound that helped hasten his departure.

Many fear that the ex-president, who has cast a large web of tribal and family relations during his 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.

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.