Unknown gunmen attacked on Sunday afternoon the local police headquarters in Yemen's southern port city of Aden, injuring at least three soldiers, a security officer said. The headquarters of Aden's criminal police investigations was attacked by unidentified assailants armed with automatic rifles, injuring at least three soldiers, who were guarding the building, the local security officer told Xinhua on condition of anonymity. "A shootout flared up between the armed attackers and security forces positioned around the police administration building and lasted more than half an hour," the officer said, adding that "the attacker managed to flee the scene after the incident." Local government authorities have tightened security measures in the port city of Aden, where secessionists are calling for a boycott of the Tuesday's one-candidate presidential election, by deploying a large contingent of army troops, backed by dozens of armored vehicles across the province, according to local residents. Leaders of the pro-seperatism Southern Movement said they would boycott the early presidential election scheduled for Feb. 21 by preventing voters from casting their ballots and calling for collective acts of civil disobedience in the voting day. The Yemeni power-sharing government has geared up for the election, which is part of a UN-backed power transfer deal to ease outgoing President Ali Abdullah Saleh out of office and pull the impoverished Arab state back from a possible civil war. Under the deal, which was signed by Saleh and the opposition in November 2011, the rival political parties nominated Vice President Abd-Rabbu Mansour Hadi as the sole presidential candidate for the early election. The anti-election riot showed the country's fragile security situation two days ahead of the elections that would end the rule of Saleh who is currently in the United States for medical treatment.