This story is still evolving and details are sketchy, but...

Apparently a Manila-based team from the Philippine National Police Special Action Force (sort of a combined SWAT/Hostage Rescue/Counterterrorist unit) went to a deeply rural section of Central Mindanao, an area under MILF/BIFF control, and tried to arrest two wanted JI terrorists, Marwan and Basit Usman. In the process, and in the middle of the night, they either directly attacked or stumbled into a camp of the MILF 105th Brigade, a unit closely associated with breakaway BIFF Leader Umeril Umbra Kato. The details are shaky, but apparently at least 30 of the police team were killed, some reports from the field saying over 50, along with much smaller numbers of BIFF and/or MILF. The difference between those is often shaky. An acquaintance from the area with MILF connections comments:

In that area, almost all major commanders from different factions are inter related by blood. An invasion force of 50 or 60 commandos is not enough to take on a heavily armed community. What is AFP going to do? Complain at the IMT? By now, the area is reinforced with different groups not controlled by the MILF. It is a blood feud, rido.

The size of the PNP team is not clear at this point, and few details of the actual mission are public (a sanitized version is probably being concocted). It does appear that there was no coordination with Philippine Army units in the area, and that when things went wrong the police team had nobody to call for help. There are still questions: the fighting apparently went on for some time; there are significant army and air force assets in the area, and it is not entirely clear whether there was any attempt to assist the group, or if not, why not.

The PNP is claiming that Marwan was killed in the fighting, but he's been killed several times before and always seems to reanimate.

This area is nominally covered by the Government/MILF peace negotiations, under which each side are expected to advise the other of operations. That clearly did not happen.

A lot more information will emerge, much of it probably distorted to serve various views. The peace negotiations, which already seems to be dead in the water, are a likely casualty.