(CNN) -- Malaysia's opposition leader has urged his supporters to wear black at a rally to be held in the capital Kuala Lumpur on Wednesday to protest Sunday's vote, which he claims "was marred with unprecedented election fraud."
"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 Ibrahim said in a statement released Monday.
Prime Minister Najib Razak, the leader of the Barisan Nasional coalition that has ruled for 56 years, took his oath of office on Monday after winning 133 out of 222 parliamentary seats, Malaysia's national news agency Bernama reported.
Anwar's Pakatan Rakyat coalition won 89 seats, handing Barisan Nasional its worst ever electoral showing.
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.
"This is entirely contrary to pre-election opinion polls, including those by Pew Research and the Merdeka Center, which showed significant support for both the Prime Minister and his ruling party and suggested they were on track for a victory."
The weeks leading up to Sunday's election saw reports of firebombs, texted death threats and beatings and there were widespread reports that indelible ink used to mark the fingers of advance voters was washing off.
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.
Investors reacted favorably to the election outcome, sending Malaysia's stock index up more than 8% on Monday and strengthening the currency.
Liam Hanlon, senior political analyst at Cascade Asia Advisors, said the government should interpret its reduced margin of victory as a warning from voters that reform is needed.
"The government must heed this warning and understand that it needs to be more forward-looking in order to survive," he said.
"It would be a shame if the opposition's strong showing actually propelled the country back into more pronounced race-based policies."
In the 1970s, Malaysia introduced policies that effectively split the country along ethnic lines, giving preferential treatment to the so-called Bumiputera, or ethnic Malays and natives of Sarawak and Sabah.
According to the CIA Factbook, just over 50% of the country is Malay. Chinese make up 23.7% and Indians 7.1%.
Anwar appealed to voters with a promise to change race-based policies to make them more inclusive and needs-based.
James Chin, a professor of political science at the Malaysian campus of Australia's Monash University, said the results showed that Malaysia's ethnic Chinese population had voted in a bloc against the government for the first time. He added that some urban Malays had also voted for the opposition.
He said that Anwar's efforts to address election irregularities would depend on whether he drew support from across Malaysia's multi-ethnic population.
"Elections in Malaysia have never really been free and fair but the big difference this time was for the first time in Malaysian history, the Chinese on both sides of the South China Sea voted together against the government," he said, referring to Peninsular Malaysia and Malaysian Borneo.
"The key to tomorrow's attendance is to see if the Malays are willing to come out. If only the Chinese come out it will look like a racial protest. It needs to be a multiracial protest if it's going to have any effect."