MANILA - Gunmen abducted and killed at least 21 people in the southern Philippines Monday, apparently to prevent a woman filing her husband's nomination to run for provincial governor in elections next year, the military said.

Lieutenant Colonel Romeo Brawner said the bodies of 13 women and eight men were found in the area where about 30 people were taken hostage.

We believe more bodies are buried, Brawner said. Unfortunately, the killing happened before our troops got there.

Some of the victims were beheaded, and bodies mutilated, local officials said.

Military officials said the dead included Genalyn Tiamzon-Mangudadatu, who was on the way to file the nomination of her husband to contest the governorship of Maguindanao province against Datu Andal Ampatuan, the head of a powerful local family.

She was accompanied by two lawyers, several members of her family and local journalists.

President Gloria Macapagal Arroyo condemned the violence and ordered her top security officials to personally oversee military action against those behind the killings.

No effort will be spared to bring justice to the victims and hold the perpetrators accountable to the full limit of the law, Arroyo said in a statement read by her press secretary, Cerge Remonde. Civilized society has no place for this kind of violence.

CLAN RIVALRIES

The southern Philippines is driven by clan rivalries, including one between the Mangudadatus and the Ampatuans. Many politicians and elected officials in the region maintain well-equipped private armies.

Ampatuan has been elected governor of Maguindanao three times previously, always unopposed, although he resigned from the post earlier this year, apparently to circumvent term limits on elected officials.

Of the 22 mayors in his province, most are sons, grandsons or other relatives. Two of his sons have been killed in violence linked to clan wars.

In a 2007 interview with Reuters, Ampatuan said people did not run against him or his family members because they had little chance of winning.

It's because of popular support, he said. Because I am so loved by the constituencies of the municipalities, they ask me to have my sons as representatives.

Arroyo has called him a valuable ally in the past. In the 2004 presidential elections, she won most of the votes in Maguindanao. In one town, her rivals did not get a single vote.

Earlier, military officials had said about 100 armed men, several of them in police uniform, had stopped the Mangudadatu convoy at a police checkpoint on a highway and taken the victims to a remote mountainous area.

Remonde, Arroyo's press secretary, said extra troops had been sent to the area to defuse tension. Checkpoints were set up to limit movement and lessen chances of retaliation.

The election process for the May 2010 national polls began last week with the filing of candidacies for more than 17,800 national and local positions.

The official campaign begins in February for candidates running for president, vice president and 12 seats in the upper house of Congress. For those seeking local positions and nearly 300 seats in the lower house of Congress, campaigning starts in late March. Polling is on May 10.

Elections in the Philippines are usually marred by violence, especially in the south, where security forces are battling communist rebels, Islamic radicals and the clan rivalries.

(Reporting by Manny Mogato; Editing by Raju Gopalakrishnan and Ron Popeski)