An armed forces spokesman said the bomb exploded during a basketball game in the town of Parang in Maguindanao province, and was aimed at killing the town mayor, Vivencio Bataga.
"The mayor was wounded but he is fine. I saw 10 people dead," said town councillor Gaudencio Teves, adding that 40 others were taken to different hospitals.
"The mayor apparently lost his teeth," said military spokesman Julieto Ando.
Regional police chief Isnadji Bantala said he had ordered all officers in the town to "secure the place and pursue the suspects" but said no one had claimed responsibility.
"This is something political. Bataga survived similar attacks in the past," he said.
Bataga is a former army colonel who is seeking re-election and police said he survived at least three attempts on his life last year.
Filipinos are due to vote in national, regional and local elections on 10 May. Campaigns in one of Asia's most turbulent democracies are often marred by violence, intimidation, vote-buying and cheating.
"We condemn this attack. We believe this is politically motivated," he said. "Bataga is believed to be the target."
The area around Parang is a stronghold of the Moro Islamic Liberation Front (MILF), the largest of several rebel groups seeking independence for the south of the archipelago.
But police played down the chances of a rebel attack. A shaky ceasefire with the MILF has held so far as the rebels inch toward formal peace talks with the government.
Eid Kabalu, a spokesman for the MILF, denied the rebels had any involvement in the attack.