Fighting between rebels and troops broke out earlier this month
Separatist rebels have launched a series of attacks on towns in the southern Philippines, amid ongoing unrest over an autonomy deal.
Guerrillas raided several towns in Lanao del Norte province, burning shops and houses. Casualties were reported, but exact numbers are unclear.
Troops were sent to fight the rebels and, in a national address, President Gloria Arroyo vowed to defeat them.
The violence came after the Supreme Court blocked a territorial deal.
The government had agreed to expand an existing Muslim autonomous zone on Mindanao island, in a bid to end years of separatist fighting.
But the court suspended the deal earlier this month after Christian communities argued it would increase sectarian divisions.
Two weeks ago rebels occupied villages in North Cotabato province. The military used air strikes and artillery assaults to force them back and casualties were reported on both sides.
Monday's raids in Lanao del Norte province took place near the borders of the existing autonomous zone.
In pre-dawn attacks, several hundred rebels from the Moro Islamic Liberation Front (MILF) raided the towns of Kauswagan, Maigo and Kolambugan, officials said, as well as outlying villages.
In Kolambugan, troops and police fought the rebels in the town centre while houses burned, Mayor Beltran Lumaque told a local radio station.
"They have taken over the business centre, the rural banks, pawnshops," he said. "We need reinforcements."
By noon the rebels were said to have left some towns, but people were fleeing affected areas, reports said.
A Philippine army general told the Associated Press news agency that the bodies of 24 civilians had been found, while reporters for the French news agency AFP said they saw nine bodies lying by the road in one village.
MILF spokesman Eid Kabalu blamed the attacks on a renegade commander.
"Our leadership has not sanctioned these attacks. This has to stop if we can confirm the involvement of our forces," he said.
In Manila, President Gloria Arroyo pledged to put an end to the violence.
"As your commander-in-chief I have ordered the armed forces of the Philippines and the Philippine National Police to defend every inch of Philippine territory against MILF forces, and immediately restore peace in the affected areas in Lanao del Norte," she said.
"We will not tolerate and will crush any attempt to disturb peace and development in Mindanao."
The president is to hold a meeting with senior advisers later in the day to discuss further action.
More than 100,000 people have been killed in almost four decades of fighting in the south.
The Philippine government had hoped that the autonomy deal would kick-start talks with the rebels. The Supreme Court is scheduled to hold another hearing on the agreement on 22 August.