Samoa's caretaker Prime Minister Tuilaepa Sailele Malielegaoi has accused the FAST Party of "treason" following the party's swearing in during a ceremony on the lawns of Parliament. 

Hundreds gathered on Monday expecting Parliament to reconvene, only to find FAST Party leader, Fiame Naomi Mata'afa, and her supporters had been locked out of Parliament. 

So the ceremony took place under a tent on the lawns outside. 

But as the ceremony concluded and Fiame sworn in as the new Prime Minister, Tuilaepa Sailele Malielegaoi, refused to stand aside. 

Rather than conceding, he accused the FAST Party of leading a coup and said he'd take action tomorrow. 

"This is treason, and the highest form of illegal conduct. None of what they did is legitimate. The Devil has won and taken over them," he said. 

Fiame has also called the conduct of Tuilaepa's Government illegal, likening it to a "bloodless coup." 

Auckland University Pro Vice-Chancellor Pacific, Dr Damon Salesa, told Newshub the political situation remains in crisis. 

"I think it's unclear what will happen next. It's not clear he (Tuilaepa) will simply concede even when another Prime Minister is sworn in and that's really troubling. 

Earlier on Monday, FAST party members arrived at Parliament to find a heavy police presence and Parliament locked. 

Samoa's Chief Justice Satiu Simativa Perese walked up the steps of Parliament, but left after not being able to open the doors. 

FAST Party supporter Nynette Sass told Newshub what unfolded was disturbing. 

"To think that they did that on this very special day is very saddening for us. All we can do is pray for peace and calm," she said.  

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Samoa's political crisis has been fast moving

On May 17, the Supreme Court found an attempt to add an extra female seat to parliament was unlawful, as was a call by Samoa's Head of State Afioga Tuimalealiifano Vaaletoa Sualauvi II for new elections to be held.

The Supreme Court ruling gave FAST a narrow 26 to 25 majority. 

On May 21, the head of state said the Parliament should convene on Monday. 

But late on Saturday night he changed his mind, suspending the session without giving a reason. 

On Sunday the Supreme Court ruled that was unlawful, saying Parliament should resume as planned.

Shortly after that Parliament's retired Speaker hit back - saying Parliament was "postponed" 

Meanwhile, caretaker Prime Minister Tuilaepa, while urging calm, told supporters his HRPP Party remains the Government, and only the Head of State has power to convene Parliament.

Dr Salesa says that's seriously concerning. 

"What he's done is just run roughshod over the conventions of Parliament and the rule of law. It's now pretty blunt and it's pretty obvious." 

Dr Iati Iati a senior lecturer in politics and international relations from Victoria University says what's occurring is "surprising but not totally unexpected" given Samoa's political system is complex and has never been tested like this. 

"What we're seeing play out in Samoa are different institutions within the Samoan political system really trying to find the parameters of their authority," he told Newshub. 

He said we wouldn't be in this position if there was a clear majority and each side is pushing the boundaries.  

He says the advent of the FAST Party could see a significant shift in geo-political relations, especially with the likes of China.

"This may be an area to keep an eye on - how will FAST negotiate their relations with key players in regional geopolitics."  

Prime Minister Jacinda Ardern urged Samoa to follow the rule of law. 

"We hold a huge amount of trust and faith in the institutions in Samoa. In the judiciary, in their democracy and of course in the outcome that the election delivered".