Fiji police have arrested 16 people over an assassination plot against the South Pacific island nation's coup leader and prime minister.

More arrests are possible and police have asked for military assistance, Fiji Police Commissioner Esala Teleni said.

'Our intelligence has received information that several suspects and others are believed to be armed. Hence my request for military assistance because the police cannot handle it on their own,' Teleni told a news conference in Suva.

Police did not release details of the assassination plot against Prime Minister Frank Bainimarama, the military chief who came to power in a 2006 coup, prompting opposition politicians to accuse his regime of tightening control of Fiji.

Police said they would lay charges against the 16 people arrested over the past two days.

New Zealand Prime Minister Helen Clark said one of those arrested, a New Zealand man, had been beaten so badly he was unable to talk and was being detained in a Suva hospital.

'It's absolutely terrifying. No person should be treated like this,' Clark told Radio NZ on Monday.

A hospital source said the man had a broken jaw and ribs.

Bainimarama seized power in a coup on Dec 5, 2006, claiming the democratically elected government of Prime Minister Laisenia Qarase was corrupt and soft on those behind a 2000 coup. Fiji has had four coups since 1987.

The self-appointed prime minister said in June he was willing to hold elections by early 2009, but many international observers said Fiji was making little movement towards democracy, with the military-backed government firmly in control.

Fiji media reported that police believe the assassination plot could be linked to foreign aid groups and neighbouring countries.

Australia and New Zealand have been critical of Bainimarama's regime, calling for a return to democracy.

'Of course the New Zealand government hasn't been involved in any such thing,' Clark told Radio NZ on Monday.Reuters