Twenty-one people found dead after group of more than 40 abducted in Mindanao.
The army has linked the killings to political rivalries ahead of next year’s local elections.
In a televised address to the nation on Tuesday afternoon, Arroyo said she was determined that those behind the killings would be held “accountable to the full limit of the law”.
The killing of political rivals and journalists is common in the Philippines, so much so that the government set up two bodies to investigate the practice.
The police-led task force Usig confirmed 116 cases of political murder between 2001 and 2007. Twelve suspects were arrested but no convictions have been made.
The Melo Commission in 2007 found 136 political murders, with strong evidence implicating the armed forces in the killing of those perceived to be enemies of the state.
“The chief of staff has ordered the establishment of check points and choke points and as of last night, the military elements were in place to preserve peace in the areas.
“Additional troops have also been deployed to the area last night to further secure the area,” she said.
Earlier officials in the president’s office said she had ordered a state of emergency in the area, following what they said was the worst political violence seen in the country in recent history.
Lieutenant-Colonel Romeo Brawner, a spokesman for the Philippines military, said that about 500 more soldiers had been sent to Maguindanao province on the island of Mindanao “to go after the criminals” believed to be behind the killings.
He said the troops were under orders to arrest the followers of Andal Ampatuan, the incumbent governor suspected of being behind the killings.
“We maintain the Ampatuans are the suspects,” Brawner told the AFP news agency.
On Monday, the military said 22 bodies – most female, some beheaded and mutilated – had been found in a mass grave in a remote mountainous area.
Al Jazeera’s Marga Ortigas, reporting from the Philippines, said Ampatuan is known to be closely-associated to the government and a close ally of Arroyo so people are watching to see what action the government will take.
Ampatuan is seen to have delivered the votes that swung the 2004 elections in Arroyo’s favour, so people in the province fear he may not be punished if he is found to be behind the killings, she said.
Meanwhile Mangudadatu, whose family members, including his wife, were among those killed according to the military, told local radio that at least four people had survived the attack.
He said the survivors were safe under his care and “will come out at the right time”.
Mangudadatu told local television network ABS-CBN on Monday that he had been warned about the dangers of standing for the governorship against Ampatuan.
Mangudadatu said he stayed behind in the capital Manila and sent his wife, Genalyn Tiamzon-Mangudadatu, to file his nomination in Maguindanao on his behalf.
Maguindanao is one of the most politically tense provinces in the country.
The governor position is hotly contested because it is the seat of the autonomous region of Muslim Mindanao, she said.
Many politicians and elected officials in the region maintain well-equipped private armies.
Leila de Lima, a former election lawyer who now chairs the Commission on Human Rights in the Philippines, told Al Jazeera that the killings showed a “breakdown of law and order” in the area.
“Those politicians who are reported to be behind this, these are the gods in the area, they feel untouchable, they think and act like they’re above the law. All these years they have been tolerated,” she said.
“We’re calling on the national authorities, including the executive department and the police, to do something swift and decisive to avert further violence and to obviate the impression that the situation might get out of control.
“We need decisive action this time. We cannot afford to have such a continuing environment and culture of impunity.”