The military regime that seized power in a 2006 coup says the constitution is another step toward free elections next year.
But many say the document curtails certain freedoms and is self-serving to the regime.
It's the country's fourth constitution since 1970.
Fiji's leader Frank Bainimarama said the constitution enshrines principles including an independent judiciary, a secular state, and a range of civil, political, and socio-economic rights.
Bainimarama plans to contest next year's elections.
Fiji police on Friday arrested about a dozen people, including a former prime minister, who were apparently peacefully protesting the new constitution, highlighting the restrictions opposition groups continue to face.