(CNN) -- The head of Malaysia's police force has warned opposition supporters they face arrest if they attend a rally Wednesday evening organized by losing candidate Anwar Ibrahim.
Inspector-General of Police Tan Sri Ismail Omar said the rally was illegal because organizers didn't apply for a permit.
"The organizer must comply with the Peaceful Assembly Act 2012. Marching without permit aimed at arousing anger is wrong according to the law," Ismail said, according to the country's state-run news agency Bernama.
According to a statement from Anwar's Pakatan Rakyat (PR) coalition, the event is being held at a private location so is not bound by the law.
The group said it had informed police but there was no need for a permit.
Anwar has slammed the outcome of Sunday's general election, which extended the 56-year rule of the Barisan Nasional coalition, giving it a simple -- albeit reduced -- majority of 133 of 222 parliamentary seats. PR won 89.
"Our conscience cannot allow us to accept election results conjured through frauds and cheating. A fight for clean and fair election remains the single most important fight that any Malaysian should relate to," Anwar said in a statement released Monday.
He urged his supporters to wear black to the rally as a sign of mourning for Malaysia's democratic process.
A government spokesperson said on Tuesday that allegations of fraud in the election process were "unsubstantiated" and the government's victory was in line with independent polling.
"Anwar claimed the only way he could possibly lose was if there was 'massive fraud'," the spokesperson said in a statement.
Prime Minister Najib Razak took his oath of office on Monday after 80% of the electorate turned out to cast their vote.
During the weeks leading up to the election, reports emerged of escalating violence, including beatings and death threats.
Human Rights Watch also claimed that cyber-attacks had been directed at a number of Malaysian news websites, restricting access to reporting on the election within the country.
Other allegations surfaced on polling day, with Anwar's party and election observers alleging the government exchanged cash for votes and flew in foreigners to cast their ballots in favor of Barisan Nasional -- a charge denied by the government.
"We were told that 40,000 foreigners were being flown across Malaysia to vote. However, passenger manifests prove that a few hundred Malaysian citizens were flown to their home states, entirely legally," it said in a statement.