A senior Philippines official says martial law has been imposed in the southern province of Maguindanao, where 57 people were killed last week in the country's worst election-related violence.

The government suspended civil rights in the province, Executive Secretary Eduardo Ermita told reporters, the first time in 28 years such a drastic measure had been imposed anywhere in the country.

Mr Ermita, reading a proclamation by President Gloria Arroyo, said the president had the authority to call in the military to suppress rebellion and protect civilians.

He added the declaration was made after the government received reports there were armed groups massing in the area to launch drastic action after a local mayor was arrested in connection with the massacre of members of a rival political clan, civilians and 30 journalists on November 23.

The patriarch of the powerful political family, Datu Andal Ampatuan, and his governor son were taken into military custody after he and several members of his clan were linked to the murders, said General Victor Ibrado, head of the armed forces.

Another son, the local mayor, was already in custody.

- Reuters