A suicide car bombing killed 26 people outside a presidential palace in southern Yemen Saturday, just hours after longtime vice president Abd-Rabbu Mansour Hadi was sworn in as the country's new leader.

Security officials say the death toll could rise, and at least 20 people were wounded by the explosion in the southern port city of Mukalla, about 400 kilometers southeast of the capital, Sana'a. Officials say the majority of those killed were presidential guards.

Mr. Hadi faces the challenge of uniting a heavily divided country. Yemen has long dealt with a separatist insurgency in the south, Shi'ite rebels in the north and an active wing of al-Qaida.

Mr. Hadi took his oath in front of parliament Saturday. In a televised speech afterwards, he pledged to continue the fight against al-Qaida militants and make it possible for displaced Yemenis to return to their homes.

He warned that if Yemen does not deal with its political and social crisis, “chaos will reign.”

Mr. Hadi was the only candidate in this week's presidential election, and he received more than 99 percent of the votes cast. He will preside over a two-year transition period that envisions parliamentary elections, a new constitution and restructuring of the military.

His election advances the transfer-of-power pressed by the United States and Gulf nations that had reduced some of the bloodshed from anti-government protests and prompted Ali Abdullah Saleh, Yemen's long-time ruler, to relinquish power. The agreement promised immunity for the former president after he stepped down from the presidency.

Mr. Saleh has been in the United States receiving medical treatment for serious injuries he suffered in an assassination attempt months ago. He arrived back in Sana'a early Saturday and is scheduled to formally hand over power during a ceremony at the presidential palace in Sana'a on Monday.