But Bayan, a prominent left-leaning group, said the law endangers the civil liberties that Filipinos won in 1986, when they ousted Ferdinand Marcos, the former president, in a non-violent revolt.In a statement, Bayan said: "This is a new dark age for human rights and civil liberties."
About 300 activists rallied near the palace of Gloria Arroyo, the president, on Sunday, carrying posters showing her image with the words, "the real face of terror". Anti-riot squads stood nearby.
Ignacio Bunye, Arroyo's spokesman, said the law could be used against anybody who would resort to terrorism, including communist fighters, religious extremists and rogue policemen or troops.
"The law's message is clear: if you are armed and kill civilians, you will be prosecuted," he said.
"The law's message is clear: if you are armed and kill civilians, you will be prosecuted"Ignacio Bunye, spokesman for Gloria Arroyo
But Bayan said it would question the law's constitutionality before the supreme court within days, arguing that its definition of terrorism was too broad and could cover legitimate dissent.
The law, signed by Arroyo in March, allows detention of suspected terrorists without charge for three days and includes "rebellion or insurrection" among crimes considered terrorism.
It has been welcomed by the US and Australia, whose officials say training camps in the southern Philippines could produce fighters who could strike anywhere in the world.
Renato Reyes, a protest leader, said the law could be used against left-wing opponents of the government.
About 800 activists have been killed in extra-judicial killings and nearly 200 others have gone missing under Arroyo's rule, he said.
The military has denied any involvement in the deaths.
However, a UN human rights investigator and a Philippine fact-finding commission created by Arroyo have linked soldiers to some of the deaths and disappearances.
Blancaflor said that legislators watered down the anti-terror bill with safeguards to prevent abuses and cited a provision that bans the torture of suspects.