The victims were travelling to register a candidate for the elections
At least 46 people are now known to have died in a mass political killing the Philippines, after more bodies were discovered in a shallow grave.
Philippine President Gloria Arroyo has declared a state of emergency in two provinces on the island of Mindanao to allow police to search for the gunmen.
The victims were killed as they were travelling to file nomination papers for elections next May.
It is one of the worst incidents of pre-poll violence in the Philippines.
There are conflicting reports about whether anyone survived.
On Monday it was announced that 21 bodies had been found. This number has now increased to 46 and the death toll from the incident may rise still further.
Vaudine England, BBC Asia analyst
The brutality of the killings in Maguindanao has highlighted the domination of Philippines politics by guns, warlords and lawlessness.
It has also changed the nature of "rido", the local word for clan wars and vendetta. Rido has long focused on election disputes or land conflicts and has involved killing, but usually of one person at a time and avoiding the killing of women or non-relatives. This incident targeted women and journalists who were not members of either powerful family involved.
Complicating the mix in Mindanao is the ongoing Muslim and communist insurgencies which have long been used by the authorities to justify new sources of funding for local favourites.
"It's a big area where these bodies were found. They are finding a couple of bodies every couple of hours or so," Interior Secretary Ronaldo Puno was quoted as saying by AFP news agency.
He described a farming area covered in hastily dug graves, several kilometres away from where the killing is thought to have occurred. A bulldozer was parked nearby.
All the victims were civilians, including the wife of the vice-mayor of Buluan, a town in Maguindanao province, who had intended to file nomination papers for her husband to run for governor in local elections next year.
An investigator at the scene told reporters all victims had been shot at close range, some in their vehicles and others apparently while fleeing.
"It is an undying fact that Philippines is still ruled by clans and warlords - and these people are monsters created by President Gloria Arroyo and past governments," said Marites Vitug, editor of Newsbreak and author of several books on Mindanao, corruption and politics.
President Arroyo: "No effort will be spared to bring justice to the victims"
Central government in the Philippines is habitually weak and buys local support by supporting the local patriarchs.
Complicating the mix in Mindanao is the ongoing series of Muslim and communist insurgencies which have long been used by the authorities to justify new sources of funding for local favourites.
In this case some of the suspected killers were "Civilian Volunteers", groups of young men organised to support the local police in "anti-insurgency" work.
"The volunteers are supposed to support the police but in the process they are used by warlords. Budgets are limited for the police so it is common knowledge that powerful local families support them financially," said Ms Vitug.
The dominance of patronage politics in the Philippines, she explained, has allowed political families to thrive.
The BBC's Vaudine England says both clans in this case are allied to Mrs Arroyo, limiting analysts' expectations of any effective response from the central government.
The country is to hold nationwide elections in May 2010. Registration for local and national races began earlier this month.
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