The attack took place in southern Mindanao on Thursday, as the group returned from a campaign event ahead of elections on 13 May.
Abdulmalik Manamparan, mayor of Nunungan town, survived the attack, but a number of his relatives were said to be among the dead.
Violent incidents often occur in the run-up to elections in the Philippines.
Four years ago, gunmen said to be from a rival clan killed 58 people, including dozens of journalists, in Mindanao as they travelled to file election papers for a local leader planning to contest polls for governor.
Police investigators said they suspected the recent attack was linked to a rival clan, the Associated Press news agency reported.
The polls on 13 May will see seats contested at national, provincial and municipal level across the nation. A presidential election is not due until 2016.