An explosion tore through a polling station in the southern Yemeni city of Aden on Monday, followed by gunfire that killed one soldier, a day before a presidential vote to replace Ali Abudllah Saleh.

The soldier was killed and another injured when unidentified gunmen opened fire on an army patrol in the same neighbourhood shortly after the blast, an official said. No one was hurt in the blast, he said.

The official could not confirm whether the two incidents were related.

"The explosion caused a big hole in the building's wall and shattered the windows of nearby houses," the official told Reuters. He said authorities were investigating both incidents.

Millions of Yemenis vote on Tuesday in an election in which Vice President Abd-Rabbu Mansour Hadi, representing Saleh's supporters and opponents, is the only candidate. A year of protests and unrest have forced Saleh to end his 33-year rule.

Southern secessionists and Houthi Shia rebels in the north are boycotting the vote but analysts say they are unlikely to be involved in violence to disrupt it. In the south a powerful branch of Al-Qaeda is active and has controlled a number of cities there for nearly a year.

Saleh handed power to Hadi in November under a power-transfer deal brokered by Gulf Arab states with the help of the United Nations, the United States and the European Union.