By Tom Westbrook

SYDNEY (Reuters) - Fijian police have released three opposition lawmakers detained over the weekend for criticizing the constitution in what Amnesty International described as a "brazen crackdown" on freedom of expression.

Police on Saturday and Sunday detained leaders of three opposition parties and three other organizers and participants involved in a forum on constitutional reform in the Pacific island's capital of Suva last week.

All have since been freed without charge, but the case has been referred to the public prosecutor, police said in a statement issued on Sunday evening.

Police said the detentions were carried out in response to "issues that could affect the safety and security of all Fijians".

There is widespread concern among Fiji's opposition parties that the constitution gives too much power to a government originally installed in 2006 through a bloodless military coup.

"I think it's politically motivated," Fiji Labour Party leader Mahendra Chaudhry told Reuters by phone shortly after he was released on Monday.

"The regime here is very sensitive to criticism. They have seen all three political parties in opposition coming together in one forum and I think they feel a bit jittery about it."

Opposition MPs wore black ribbons at Monday's opening of parliament and some boycotted the ceremonial session in protest over the detentions.

A spokeswoman for Fiji's Director of Public Prosecutions declined to comment. A spokesman for Prime Minister Frank Bainimarama did not immediately respond to a request for comment.

After seizing power in 2006, Bainimarama stood down from the military to run as a civilian in the country's 2014 elections, winning by a landslide.

His opponents say the constitution, drawn up in 2013, concentrates too much power in the prime minister's office.

Jone Dakuvulua, who organized the reform forum and chairs non-government organization Pacific Dialogue, said his arrest highlighted some of these problems.

"It's an indication that even though we have a so-called parliament, we do not have democracy," Dakuvulua told Reuters by phone.

Rafendi Djamin, Amnesty International's director for Southeast Asia and the Pacific, said the detentions were "a brazen crackdown on people for peacefully exercising their right to freedom of expression".

(Reporting by Tom Westbrook; Editing by Nick Macfie)