The leader of the Fiji Labour Party has called on the interim government to revoke the Public Order Act.
The act was published on Monday but the military-led government says it was enacted on Friday.
Mahendra Chaudhry says he is shocked at the decree, which he describes as highly repressive and worse than the Public Emergency Regulations it replaced.
The new act gives the military and police vast new powers and bans legal recourse for any action carried out under it, Radio New Zealand International reports.
Mr Chaudhry says the measures are unheard of, put Fiji on an altogether different level, and are likely to damage its already ailing economy.
He says it is meaningless for those in power to compare the act with that in other countries.
"They forget that these countries have a functioning democracy, they have a parliament, they have a free media.
"In Fiji, all this is not possible - we don't have a parliament and therefore nobody is accountable for their actions, except the military itself."
The interim government says the act is to prevent unrest of the kind seen in 2000, which led to the coup that ousted Mr Chaudhry.