Party president Abd Al-Hadi Awang said he would challenge the results of last weekend's elections in court on Wednesday.
He also brushed off suggestions that PAS's defeat on Sunday meant Muslims had rejected plans to turn Malaysia into a theocratic state.
He blamed it squarely on alleged irregularities in the electoral rolls and a "conspiracy" by Prime Minister Abd Allah Ahmad Badawi's ruling National Front coalition, involving rigging of the results and rampant bribery to buy votes.
The Alliance for Reform and Democracy in Asia - a regional election monitor based in Singapore – said on Monday that its team of observers had "detected irregularities that have disadvantaged the opposition."
"The election was not free and fair," said spokesman Gandhi Ambalam.
Election results questioned
PAS retained control in its northern stronghold state of Kelantan with the slimmest of majorities after a recount, but lost power in neighbouring Terengganu and was thumped in elections to the federal parliament.
"We cannot accept the results. It was won with phantom hands working behind the scene.
"We have concrete evidence of cheating and manipulation by the Election Commission (EC) together with the National Front," Abd Al-Hadi told reporters after chairing a five-hour party meeting.
"We cannot accept the results. It was won with phantom hands working behind the scene"
Abd Al-Hadi Awang,
PAS party president
He said the high voter turnout, more than 90% in some areas in Terengganu, was illogical and PAS had proof the EC intentionally issued two electoral rolls to confuse voters.
PAS workers also found names of the same voters in rolls in several centres and fake addresses.
Sticking to policy
Abd Al-Hadi said the recount of votes also highlighted irregularities, because some opposition candidates who lost on the initial count later won a seat after several recounts.
He said the party would stick to its struggle for an Islamic state.
"We will not shift from our Islamic principles. Our image will remain the same. It is not PAS's mistake. We are more used to losing than winning, so there is no need for us to change.
"What needs to change is the election laws and a reform of the EC."
PAS secretary general Nashar al-Din Isa said the party would file a petition in court soon to contest the results.
He acknowledged that PAS's Islamic ambitions might have turned off some voters and that it was prepared to lose some seats in Kelantan and Terengganu, but said the ruling coalition's landslide was suspicious.
On Tuesday, PAS lodged a complaint with the Malaysian Human Rights Commission (SUAHAKAM) demanding new elections and calling for the formation of a cabinet to be suspended.