The grenade exploded behind a mosque in the compound of Andal Ampatuan Snr in the southern city of Shariff Aguak late on Sunday, said provincial police commander Senior Superintendent Alex Lenesis.
A few minutes later, a mortar shell exploded near a petrol station in the same city but both blasts were small and no one was hurt, said Lenesis.
The police said the attacks could have been carried out by the Moro Islamic Liberation Front, a Muslim rebel group which is a longtime enemy of the Ampatuan clan.
"We suspect it was the work of Milf rebels out to get even with the Ampatuans," Lenesis said, but added that police were not ruling out "lawless elements" outside the Milf.
Ampatuan Snr is the patriarch of a powerful Muslim political clan being investigated for its role in the massacre of 57 people in Maguindanao province on November 23.
The murders, allegedly carried out to keep a rival from running against one of his sons in elections this year, shocked the world and prompted President Gloria Arroyo to declare martial law in the province from December 4 to 12.
Several Ampatuans, including Andal Ampatuan Snr, have been arrested in connection with the massacre. Amputuan Jr is due to go on trial on Tuesday charged with multiple counts of murder.
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