There was no claim of responsibility for the bloodshed in the predominantly Muslim region wracked by political tensions between rival clans.
The convoy of vans carrying about 40 people was hijacked in Maguindanao province, about 560 miles south of Manila, and army troops later found the bullet-riddled bodies of 13 women and eight men, regional military commander Maj. Gen. Alfredo Cayton said.
It was unclear if anyone survived the attack. An army and police search was under way for the other hostages.
The National Union of Journalists of the Philippines said at least 10 local reporters were part of the convoy. Their organizations 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.
The politician, Ismael Mangudadatu, was not in the convoy and said his 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 relatives were among the dead.
Victims' relatives blamed political rivals in national elections slated for May 2010.
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 "unequaled 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."
About 100 gunmen were involved in the hijacking, military spokesman Lt. Col. Romeo Brawner said.
Also in the convoy were Mangudadatu's two sisters, followers and several local journalists. They were traveling to nearby Shariff Aguak township to file Mangudadatu's nomination papers for the position of governor of Maguindanao province, Brawner said.
Mangudadatu, vice mayor of Buluan township, accused political rivals belonging to a prominent clan for the massacre. Representatives of that family did not respond to the allegations.
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.