KUALA TERENGGANU (Malaysia) - FIVE Malaysian opposition members were arrested in north-eastern Terengganu state on Monday as they campaigned for a hotly contested by-election, officials said.

The opposition Keadilan party said the group, including its deputy youth chief Faris Musa, were detained for putting up posters linking Deputy Prime Minister Najib Razak with the high-profile 2006 murder of a Mongolian woman.

'Five people were arrested and released on police bail,' said Home Minister Syed Hamid Albar, adding that the five were suspected of defamation.

'You can't breach the law. If they spread criminal defamation the police should take action. In a democratic system we should act with responsibility,' he told a press conference.

Keadilan officials said the poster featured a photograph of Mongolian woman Altantuya Shaariibuu, the lover of a close associate of Mr Najib, whose body was blown up with military-grade explosives in a jungle clearing.

Mr Najib's associate Abdul Razak Baginda was last October cleared of involvement in the case, but two police officers from an elite unit which guards the prime minister and his deputy are yet to mount their defence.

The incident occurred in the run-up to a Jan 17 by-election in the state capital Kuala Terengganu for a parliamentary seat that will not alter the balance of power, but which will deliver a big boost to the winning side.

The government is striving to show its reform credentials after a disastrous setback in general elections last year that forced Prime Minister Abdullah Ahmad Badawi to agree to hand over to Mr Najib next March.

For the Keadilan-led opposition, the vote will serve as an indication of whether voters approve of their work in the months since they won control of five states and a third of the national parliament.

Campaigning officially kicks off on Tuesday when the two sides will announce their candidates. -- AFP