* Opposition leader and incumbent PM both claim office

* Head of State overrules court order to reconvene parliament

* NZ leader Ardern calls for election result to be respected (Recasts and updates throughout with swearing in ceremony, FAST party quotes)

By Jonathan Barrett and Colin Packham

A series of twists and turns since an April election gave the FAST opposition party a one-seat parliamentary majority has culminated in a power struggle between the courts and the head of state in the Pacific nation, a supporter of China in recent years.

FAST leader Fiame Naomi Mataafa was set to become Samoa’s first female prime minister after the country’s top court upheld the election result against a challenge supported by incumbent Tuilaepa Sailele Malielegaoi.

However, Samoa’s head of state, Tuimalealiifano Vaaletoa Sualauvi II, made the rare move on the weekend of suspending the parliamentary hearing scheduled to swear in the elected members on Monday. The government supported that suspension, declining to abide by a subsequent ruling from the Supreme Court that the swearing in ceremony should go ahead.

“Democracy is inseparable from human rights which are inalienable by our laws as well as by international covenants that we have sworn to uphold,” FAST deputy leader Laaulialemalietoa Polataivao Schmidt said on Facebook on Monday.

“Democracy must prevail, always.”

Tuilaepa told reporters in the capital Apia on Monday that only the head of state could convene parliament in the nation of 200,000.

“We remain in this role and operate business as usual,” he said.

Samoa has been a close ally of China during Tuilaepa’s more than two-decades rule as prime minister.

Fiame is expected to reframe Samoa’s relations with China after telling Reuters last week she would shelve a $100 million Beijing-backed port development, calling the project excessive for a small country already heavily indebted to China.

Fiame, a former deputy prime minister who split with the government last year after opposing changes to Samoa’s constitution and judicial system, said she wanted to retain good relations with both Beijing and Washington.

Her supporters gathered outside parliament early on Monday, singing songs from Samoa’s independence movement more than 50 years ago, reported local media.

FAST members then gathered in a tent outside the locked parliament to confirm the new government members in a ceremony that Tuilaepa described as “treason”, local media reported.

New Zealand Prime Minister Jacinda Ardern said on Monday the results of the election should be respected.

“All we are doing here is calling for the outcome and the wishes of the people of Samoa to be upheld and that’s obviously the work the judiciary is doing right now,” Ardern said.

(Reporting by Jonathan Barrett and Colin Packham; Editing by Michael Perry and Jane Wardell)