SYDNEY/BRISBANE — Papua New Guinea's lawmakers gathered on Tuesday for a crucial sitting of parliament, with opponents vowing to topple the government of Prime Minister Peter O'Neill, who clings on despite promising to resign three days ago.

Additional police patrolled the capital of Port Moresby ahead of the session at which, O'Neill's opponents say, they will command a majority they plan to demonstrate with a vote to change the speaker of the house.

"(This) is enough for us to move into government," deputy opposition leader Timothy Masiu said by text message from the Laguna Hotel, where the opposition has been based for weeks, as he prepared to leave for parliament.

It was not immediately clear how that would proceed, or if the opposition would then be able to carry out its plans for a no-confidence motion, a procedure O'Neill has challenged in court.

O'Neill is resisting calls to formally quit before the challenge is heard on Friday. His spokesman said he expected a rowdy question-time "but not much else" on Tuesday.

Powes Parkop, a lawmaker who has stayed loyal to the government, said he was confident the speaker would not change.

Instability is not unusual in the South Pacific archipelago and it has not held back mining and energy investment in the resource-rich nation.

But protests over the benefits failing to reach poverty-stricken rural areas have dogged the government, sapping support from the ruling coalition.

O'Neill himself resisted calls to resign for weeks but said on Sunday he would quit in favor of Sir Julius Chan, a two-time former national leader. — Reuters