Samoa's Head of State has cancelled Monday's scheduled sitting of Parliament without explanation, in an extraordinary intervention that deepens the political turmoil seen for more than a month.

In a brief proclamation issued late on Saturday night, Tuimalealiifano Vaaletoa Saluavi II said he was suspending Parliament "until such time as to be announced and for reasons that I will make known in due course".

There appeared to be no insight to when that explanation might be, although it appeared to catch many by surprise, even in an election that has had an unprecedented number of historic twists and turns.

With religious Samoa largely shut down on Sundays, the Saturday night proclamation amounted to a last-minute intervention to cancel Monday morning's sitting, the first since the April 9 election.

There, the FAST party of Fiame Naomi Mataafa was expected to secure a majority of seats, overthrowing the long-ruling Human Rights Protection Party and making Fiame Samoa's first female Prime Minister.

The Head of State only agreed to let Parliament sit last week. He had earlier ordered a second election, which was due to be held last Friday. 

But that election was cancelled after the Supreme Court ruled his call was unconstitutional.

Saturday's proclamation also raises constitutional issues, as Parliament is meant to convene within 45 days of an election; any sitting beyond Monday would appear to be in breach of that.

Fiame and the caretaker Prime Minister, Tuilaepa Sailele Malielegaoi, were yet to issue any kind of statement on Saturday night, with the country now well engulfed in a constitutional crisis.