The centrist National Federation Party (NFP) said police had taken its leader Birman Prasad into custody on Saturday, as well as the trade unionist Attar Singh.
NFP official Prem Singh said no charges had been laid but added that the arrests came in response to a forum on Wednesday to debate the country's 2013 constitution.
"The arrest is a threat to public assembly and we don't know why they are being arrested, they are only doing their jobs," said Singh. The NFP said it anticipated Prasad might be held by police overnight.
The party said its offices and Prasad's home had been searched, with a laptop computer seized from the office.
In a statement, the center-left Sodelpa opposition party said its leader Sitiveni Rabuka, a former prime minister, had turned himself in to police.
Sodelpa also said it understood its leader would be held overnight. Rabuka was involved in two previous coups in 1987, before he was prime minister.
A link to the party's Facebook statement, posted with the tweet, no longer appeared to be working.
The news network Fiji TV also reported that police interviewed academic and politician Tupeni Baba.
All of those questioned were said to have been involved in Wednesday's constitutional panel discussions. Police were also reportedly holding Jone Dakuvula, an official of a non-government body Pacific Dialogue, which organized the event.
It remained unclear on Saturday evening whether any of the men had been formally arrested.
'Waste of money' holiday
Broadcaster Radio New Zealand International said it understood that the leader of the Fiji Labour Party, Mahendra Chaudhry, was also wanted by police.
The discussion about the constitution coincided with the tiny nation's first public holiday for Constitution Day. Speaking to the Australian Broadcasting Corporation the day before it, Prasad had described the holiday as a "waste of money" and government "propaganda".
Fijian Prime Minister Frank Bainimarama came to power in a bloodless military coup in 2006 and stood down from the military to run as a civilian in the country's 2014 elections, which he won by a landslide.
The 2013 constitution is unpopular in some quarters because it is felt to favor large parties and exclude smaller ones.
rc/jm (Reuters, AFP)