The two gunmen - armed with an M16 assault rifle and a .45 calibre pistol - had stationed themselves on a nearby bridge, Manila police chief Pedro Bulaong said.

They also fired at a passing bus, wounding the conductor and a passerby.

Presidential press secretary Ignacio Bunye condemned the murder as 'a show of force that must be met with full retribution by the authorities'.

'Criminals or terrorists will not get away with this,' he said.

'The president's orders are to get the killers and bring them to justice, and for all police outposts to be on the alert.'

Martinez gained a high profile after successfully blocking protesters from storming the palace, most recently during mass rallies in support of deposed president Joseph Estrada in the aftermath of May's presidential election.

The communist New People's Army distanced itself from the killing after a caller to a radio station earlier claimed the rebel movement had ordered the execution.

The movement's Roger Rosal said the killing 'is not our style', referring to the shots that were fired into the chest of Martinez, instead of the group's trademark single bullet to the head.

Rosal said as far as he knew, 'Martinez had no blood debt to the people'. 'Maybe he often blocked [protesters], but that's not a capital offence,' he said.

Martinez was a veteran policeman who rose through the ranks. Chief Superintendent Bulaong described him 'as a very hard-hitting police officer who ran after criminal elements'.

Manila police now have their hands full trying to solve a rash of prominent murders.

Jose Villanueva, the grandson of former Central Bank governor the late Roberto Villanueva, was stabbed to death aboard a bus on September 24 after refusing to part with his cell phone.

Two days later, Arturo Tabara - chairman of a breakaway communist faction that had signed a peace accord with the government - was killed outside a shopping centre in a Manila suburb. The New People's Army later claimed responsibility.