An unidentified man hurled the grenades into the mosque on the southern island of Mindanao.
The mosque was located inside the compound of the government's National Irrigation Administration in the town of Midsayap.
The suspect fled as the grenades detonated. Most of the casualties are employees of the government agency.
Police named no immediate suspects or motive for the attack.
Midsayap town, which is mainly Christian but has a sizable Muslim minority, has been hit by bomb blasts before.
Most of the previous attacks were largely blamed on Muslim separatist guerrillas.