Twenty-one politicians and journalists who were abducted in the southern Philippines have been found dead.
The group was seized on the island of Mindanao by armed men as they tried to file nomination papers for a candidate in local elections next year.
More members of the group are missing, feared dead.
Elections in the Philippines are often marred by violence, particularly in the south, where clashes connected to local rivalries and insurgencies erupt.
The country is to hold nationwide elections in May 2010. Registration for local and national races began earlier this month.
Jesus Dureza, adviser to President Gloria Arroyo in the volatile Mindanao region, said it was "a gruesome massacre of civilians unequalled in recent history".
He recommended that a state of emergency be imposed in the area.
In a statement, Mrs Arroyo condemned the violence and said no effort would be spared to find those responsible.
"Civilised society has no place for this kind of violence," she said.
According to local reports, the group was abducted early on Monday while on its way to an election office in Maguindanao province to file nomination papers for a local mayor, Ismael Mangudadatu.
Vaudine England, BBC Asia analyst
Election violence is not unusual in the Philippines but the scale of this attack is shocking. Every election period features assassinations of rivals, particularly in provincial areas where the forces of law and order are often tightly connected to local clans.
Every local politician has some form of personal security which, in some areas, balloons to private armies of scores or hundreds of well-armed, unregulated gunmen. In this case, the Mangudadatu and Ampatuan clans were not always at war - but the Mangudadatu family's bid to run for governor appears to have provoked a dramatic rise in tension.
It remains unclear who was responsible, but these killings are unusual for including women and journalists.
At this stage the violence does not appear to be related to the broader Muslim insurgencies in Mindanao geared toward securing more autonomy from the central government. Power and money - control over lucrative local interests - seem the most likely motives.
Mr Mangudadatu was reportedly planning to challenge local clan leader Datu Andal Ampatuan for the governor's office in the mainly Muslim province.
He was not part of the group but his wife, lawyers, aides and journalists were among those abducted when their three-vehicle convoy was attacked.
Lt Col Romeo Brawner, a spokesman for the Philippines armed forces, told the BBC the military had recovered 21 bodies, 13 female and eight male.
"Some of them have been mutilated - there are signs of mutilation. However there are also reports of beheading, but we have not yet received any confirmation about this," he said.
According to reports, the group that set off numbered between 30 and 40 people in total. It is not clear whether anyone survived the attack.
Lt Col Brawner said it was believed more bodies had been buried, and that his forces were trying to locate them.
It appeared the violence was linked to local political tensions, he said.
"This is not the first time that we are experiencing this kind of violence related to politics or related to the elections that we're going to have next year," he said. "However, this is one of the bloodiest that we have experienced so far."
Mr Mangudadatu told the Associated Press news agency that his wife and relatives were among the dead.
He said his wife, Genalyn Tiamzon-Mangudadatu, had telephoned him shortly before the group was abducted.
"She said... they had been stopped by 100 uniformed armed men... then her line got cut off," he said.
The National Union of Journalists of the Philippines (NUJP) said 12 of the 21 killed were reporters, and called for justice for the victims.
"This is a direct challenge to our efforts to strengthen democracy in this country," AFP news agency quoted NUJP spokesman Jaime Espina as saying.
International press freedom group Reporters Without Borders condemned what it called "an incomprehensible bloodbath".
"Never in the history of journalism have the news media suffered such a heavy loss of life in one day," the group said in a statement.
Elections can be particularly violent in Maguindano and other parts of Mindanao island.
Both communist rebels and separatist Muslim insurgents are fighting government troops there, and local politics can be dominated by strongmen backed up by private militias.
Vaudine England, former BBC reporter in the Philippines, says both clans in this case are allied to Mrs Arroyo, limiting analysts' expectations of any effective response from the central government.