Attackers in Yemen blew up a polling station and nearby gunfire killed a soldier in the rebellious south Monday, one day before Yemenis cast votes for a new president in a one-candidate election.

The violence in the port city of Aden underlines the security vacuum in the Arab world's poorest country after a year-long popular uprising drove longtime President Ali Abdullah Saleh from power.

Tuesday's poll culminates a U.S.-backed deal brokered by Yemen's Gulf neighbors to end Mr. Saleh's 33-year rule and replace him with Vice President Abed Rabbo Mansour Hadi.

Thousands of people attended an electoral rally in the capital, Sana'a, early Monday to support Hadi. But the fact that he is the sole candidate under the power transfer plan has raised concern about a low turnout that would curb his legitimacy. Hadi was selected as the consensus candidate by Mr. Saleh's ruling party and the opposition coalition.

The Yemeni president is the fourth Arab autocrat forced to leave office by the wave of Arab Spring uprisings over the past year. Mr. Saleh is now in the United States for further treatment of burns suffered in a June attack on his presidential compound.

Prospects for a stable, democratic transition in Yemen remain uncertain given Mr. Saleh's vow to return home to lead his party anew, a split in the country's powerful military, al-Qaida militants and a regional secessionist movement in the south and a Shi'ite Muslim revolt in the north.

On Sunday, U.S. President Barack Obama offered support for Hadi and said Yemen could be an example of peaceful political transition for other Arab nations.

The message was conveyed by U.S. counterterrorism chief John Brennan, who presented a letter from Mr. Obama to Hadi during a meeting in Sana'a.

Brennan's two-day visit was meant to ensure that cooperation with Yemen's government in fighting al-Qaida's regional branch would continue under the country's new leadership.