The reason is that several infrastructure projects, intended to combat flooding in the often storm-ravaged country, have been substandard, incomplete or simply not implemented.
Several politicians and civil servants are accused of receiving bribes to help certain construction companies win public tenders for the projects.
Since the scandal came to light a few months ago, several demonstrations, organized by various groups, have taken place. But Sunday's gathering is the largest yet, with more than 650,000 members of the religious group in attendance.
The Iglesia ni Cristo claims to have around three million members and is seen as an influential movement in the country. They vote unanimously and are often courted by political candidates before elections.
Flood mitigation measures are a particularly sensitive issue in the Philippines, which is one of the most vulnerable countries in Asia to extreme weather and typhoons, resulting in floods and landslides.
So far in November alone, two typhoons have claimed over 250 lives, and caused damage that affected millions of people.
