MANILA - Police were on a heightened security alert in Metro Manila on Friday to prevent a "spillover" of an anticipated violent aftermath of the massacre of 57 people in Maguindanao.

Director Roberto Rosales, Metro Manla police chief, said he directed all district police offices in the metro to "further strengthen security" to ensure peace and prevent the anticipated spillover of violence in the capital region.

Rosales said he has particularly ordered the Manila Police District (MPD) to set up checkpoints around the National Bureau of Investigation (NBI) headquarters located along Taft Avenue, where Datu Unsay Mayor Andal Ampatuan Jr. is detained.

The Department of Justice is readying multiple murder charges against Ampatuan Jr., who is being tagged as the brains of the allegedly election-related massacre.

Many of the victims were members and supporters of the Mangudadatu clan, including the wife of Buluan Vice-Mayor Esmael Mangudadatu, who is opposing the Ampatuans in the gubernatorial race in the province.

Rosales said he ordered Magtibay to closely coordinate with NBI Director Nestor Mantaring and discuss possible augmentation of security around the bureau's headquarters.

He said the MPD has also been ordered to increase mobile and beat patrol in the vicinity of the NBI compound.

Rosales said he has also told the MPD to put on a 24-hour standby its Special and Weapons Tactics (SWAT) unit for immediate deployment.

An ABS-CBN News report said four armed men on board two vehicles tried to enter the NBI compound on Thursday night after Ampatuan Jr.'s arrival from Maguindanao province.

The NBI said the four men's firearms were licensed, but they did not allow them to enter the compound.

Meanwhile, a certain Inspector Musa Umpan was disarmed by NBI agents after he also tried to enter the compound before Ampatuan Jr.'s arrival from Maguindanao.

Umpan claimed he was an aide of Maguindanao provincial administrator Nori Unas.

The two incidents have prompted the NBI to deploy at least 30 heavily-armed agents inside the compound.