A media event went horribly wrong when around 100 gunmen hijacked an election convoy carrying journalists, and family and supporters of a provincial governor candidate in the Philippines on Monday.

At least 21 in what a presidential adviser called the most "gruesome massacre of civilians" in recent history.

Some of the bodies were beheaded, according to Filipino media.

The convoy of vans carrying about 40 people was hijacked in Maguindanao province, about 900 kilometres south of Manila, and army troops later found the bullet-riddled bodies of 13 women and eight men.

It is thought the daytime abductions were politically motivated and that the gunmen were loyal to the province's incumbent governor.

Local politician Ismael Mangudadatu wanted to run for governor of Maguindanao province.

Mangudadatu had received threats he would be killed if he filed candidate nomination papers himself so did not travel in the convoy instead sending his wife and sisters to file the papers, thinking "that women would have some protection," reported CNN.

Mangudadatu's wife called him by mobile phone shortly before she and her entourage were abducted.

"She said ... they were stopped by 100 uniformed armed men ... then her line got cut off," he said.

He said his wife and two sisters were among the dead.

The National Union of Journalists of the Philippines said at least 10 local reporters were part of the convoy.

Their organisations failed to reach them, leading them to conclude they too were killed.

"Never in the history of journalism have the news media suffered such a heavy loss of life in one day," Paris-based media watchdog Reporters Without Borders said in a statement.

"The frenzied violence of thugs working for corrupt politicians has resulted in an incomprehensible bloodshed," it said.

It was unclear yet if anyone survived the attack.

An army and police search was under way for the other hostages.

There was no claim of responsibility for the bloodshed in the predominantly Muslim region wracked by political tensions between rival clans.

Mangudadatu, vice mayor of Buluan township, accused political rivals belonging to a prominent clan for the massacre.

Philippine elections are particularly violent in the south because of the presence of armed groups, including Muslim rebels fighting for self-rule in the predominantly Roman Catholic nation, and political warlords who maintain private armies.

The decades-long Muslim insurgency has killed about 120,000 people since the 1970s.

But a presidential adviser, Jesus Dureza, said Monday's massacre was "unequalled in recent history."

"There must be a total stop to this senseless violence," he said, recommending a state of emergency be imposed in the area to disarm all gunmen.

"Anything else will not work."

Maguindanao is part of the Autonomous Region in Muslim Mindanao, which was created as part of a 1996 peace agreement with a large Muslim rebel group.

Army troops went on full alert in Maguindanao to prevent retaliatory killings, Cayton said.