Bainimarama announced this week that the former British colony would scrap the existing flag, which has the Union Jack in the top left corner, and replace it with a design "instantly recognisable" as Fijian.
The main opposition Sodelpa Party said Bainimarama, a one-time military coup leader who won elections last year, had acted without consultation on an issue of national importance.
"In a dictatorship, you can afford to order people around, and this regime has done that many times," party spokesman Peter Waqavonovono said.
He said many people were against changing the flag, which has been used since Fiji gained independence in 1970.
"Although there are segments of our population who are bitter about our British past, we all do not share the same sentiments and therefore cannot be forced to accept their ridiculous laws," he said.
Fiji Labour Party President Lavinia Padarath said the Pacific nation's fledgling parliament was the proper forum to debate such a change.
"Bainimarama should realise that in a parliamentary democracy issues of national importance should be approved by the house (parliament)," she said.
"The people of Fiji must decide whether they want the flag changed and if so, what the changes should be."
Bainimarama this week said the flag was outdated and a national competition would be held to find a replacement.
He said the aim was to hoist a new banner on October 11 this year, the 45th anniversary of independence.
Bainimarama scrapped the Queen's Birthday public holiday in 2012 and also removed Queen Elizabeth's head from the country's currency when he still ruled by military decree.
Another former British colony, New Zealand, will hold a binding referendum next year on changing its flag, which also features a Union Jack.
New Zealand's conservative Prime Minister John Key favours a design featuring a silver fern similar to the one worn by the All Blacks rugby team.