By Simon Akam

FREETOWN (Reuters) - Sierra Leone's main opposition party said on Wednesday it would boycott parliament to protest against alleged fraud in an election that handed incumbent president Ernest Bai Koroma a second term.

The November 17 election was a test of the West African state's recovery since a civil war ended a decade ago, and international observers had said the vote appeared mostly free and fair.

"The observers were only looking at the process. We want more than the process, we want the institutions concerned," said Banja Tejan-Sie, the secretary general of the SLPP opposition party.

He told Reuters he believed the judiciary and the police helped rig the poll. "When institutions collude it is very difficult for the eye to pick, because it is cleverly done," he added.

Sierra Leone is one of the world's least developed countries but has drawn billions of dollars of state revenues from minerals and agriculture since its war.

The country's electoral commission announced last week President Koroma took 58.7 percent of the vote, enough to secure his re-election without a run-off against SLPP rival and former junta leader Julius Maada Bio.

The electoral commission has yet to release compiled results for parliamentary elections, but the SLPP held 45 of 124 seats in the assembly before the poll. A local newspaper said SLPP lost three of those seats.

Alongside the boycott, the SLPP is demanding an international investigation, the nullification of political cases in local courts and the release of its supporters and members in police custody.

Al Shek Kamara, assistant inspector general of the Sierra Leone police, told Reuters about 34 people had been arrested during the election process "all of whom have either been charged to court or released on bail pending further investigations".

President Koroma's spokesman Unisa Sesay said the November 17 poll had received "world-wide approval", but the opposition had a right to boycott the assembly. "This is a democratic country, it is their right not to go," he said.

(Editing by Richard Valdmanis and Andrew Heavens)