MANILA, May 13 (Xinhua) -- At least three explosions rocked city of Cotabato and neighboring Maguindanao province in the southern Philippines a few hours before a national midterm election, military and police authorities said on Monday.

Lieutenant Teofisto Ferrer of the Cotabato City police said the first explosion occurred around 10:15 p.m. local time Sunday night just outside the Cotabato City hall compound.

Ferrer said the explosion was caused by a mortar fired by a still unknown suspect using a grenade launcher.

The second explosion happened around 1 a.m. local time on Monday when a fragmentation grenade exploded through a municipal hall in Datu Odin Sinsuat town in Maguindanao province.

Several hours later, an explosion, caused by a still undetermined type of explosive, was recorded also in Datu Odin Sinsuat town of Maguindanao around 7:20 a.m. local time, according to Noel Detoyato, Public Affairs chief of the Armed Forces of the Philippines.

No one was killed or injured in all the incidents until now.

The Philippine military and police are on the alert to ensure that the conduct on the elections will be peaceful and orderly.

More than 60 million Filipino voters nationwide are expected to elect new senators, members of House of Representatives, and local officials in Monday's midterm elections.

Up for grabs are over 18,000 posts with around 43,000 candidates vying for the limited number of seats for senator, representative to the House, provincial governor, vice governor, and board members, city and municipal mayor, vice mayor, and councilors in this southeast Asian country.