By Elias Biryabarema

KAMPALA (Reuters) - Ugandan police said on Tuesday that a senior officer wounded in an attack by gunmen on a police headquarters in the north on Sunday had died of his wounds, and a witness said security had been stepped up in the area.

Northern Uganda is still emerging from a devastating civil war between the military and Joseph Kony's Lord's Resistance Army (LRA), which was driven out of the country about 10 years ago.

The death of the police officer raised the death toll to two in Sunday evening's attack on a regional police headquarters in Gulu town, 350 km (220 miles) north of the capital Kampala.

A soldier was killed during the attack and police spokesman Fred Enanga said Moses Masaba, who was in charge of the station when the gunmen struck - died on Monday evening.

"He was a senior standby officer at the station that night when the attack occurred," Enanga told Reuters.

Ugandan security officials say they believe the gunmen were trying, unsuccessfully, to rescue a man detained at the police station on murder and terrorism charges.

Enanga said six suspects believed to have participated in the attack had been arrested on Monday, including the driver of a pick up truck used in the assault, and two commanders.

A local newspaper journalist in Gulu said sporadic gunfire rang out on Monday evening. Businesses hurried to close earlier than normal while tens of security personnel and heavy military vehicles patrolled the town.

Enanga said the gunfire heard on Monday was from security personnel attempting to arrest suspects. "There was some form of resistance," he said, adding that the search was continuing.

Sunday's attack came amid political tensions in Uganda since veteran leader President Yoweri Museveni was declared winner of a February presidential election. Scores of people have been arrested since then, the opposition says, and protesters have frequently clashed with police.

Museveni's main opponent, Kizza Besigye, who came second with 35 percent of the vote, was arrested in May and charged with treason. He faces a maximum penalty of death upon conviction. The opposition says the accusation is trumped up.

(Reporting by Elias Biryabarema; editing by George Obulutsa and Dominic Evans)