Tens of thousands of Malaysians, dressed in black, have gathered to denounce the country's elections which they claim were marred by fraud by the coalition that has ruled for 56 years.

Malaysia's opposition leader is leading demonstrations against alleged electoral fraud in Sunday's election.

I'm here to support democracy. I feel the election is so unfair and there are so many dirty tricks - Tan Han Hui, university student

Anwar Ibrahim, head of the opposition Pakatan Rakyat, or People's Pact, called for support from people after he lost the race to become prime minister

The gathering of at least 50,000 people according to AP news agency was being held in a stadium outside the capital Kuala Lumpur, with protesters spilling out of the stands and onto the football field.

Ibrahim addressed the crowd, thanking them for their support.

He said he was grateful to the crowd for "showing the courage and

conviction to change the course of history'' in Malaysia.

"This is the beginning of a battle between the people and an illegitimate, corrupt and arrogant government,'' Anwar told the cheering audience.

Al Jazeera's Florence Looi, reporting from Kuala Lumpur, said that people had turned up in black.

"They're wearing black to symbolise a black day for democracy. The day they said when the election results were stolen from the people," she said.

"I think they should redo the election," said university student Tan Han Hui. "I'm here to support democracy. I feel the election is so unfair and there are so many dirty tricks."

Meanwhile, the White House, on Wednesday, also called on Malaysia to investigate the claims and congratulated incumbant Prime Minister Najib Razak of the Barisan Nasional (National Front) coalition.

"We note concerns regarding reported irregularities in the conduct of the election, and believe it is important that Malaysian authorities address concerns that have been raised," said Jay Carney, the White House spokesman.

"We look forward to the outcome of their investigations."

Electoral fraud allegations

Anwar has called for a "fierce" campaign for electoral reform and said the opposition would soon produce evidence backing its claims that Prime Minister Najib Razak's government committed fraud.

Najib, who was sworn in on Monday after his Barisan Nasional, or National Front, coalition, insisted the polls were totally free and fair.

A joint report released on Wednesday by two independent election watchdogs disputed that claim, saying the elections were marred by bias and irregularities that added up to "serious flaws" in the electoral system.

The report by the Institute for Democracy and Economic Affairs (IDEAS) and the Centre for Public Policy Studies (CPPS) cited continued questions over the integrity of the electoral roll.

The election was "only partially free and not fair," it said.

Lost chance

The vote was touted as the first in the country's history in which the opposition had a chance to unseat the ruling coalition, which has held a tight grip on power since independence in 1957.

Barisan retained a firm majority in parliament despite winning less than half of the popular vote, a factor partly blamed on self- serving gerrymandering and redistricting by Barisan over the years.

Anwar, who has battled the government ever since he was ousted from its top ranks in 1998 and jailed for six years on disputed sex charges, has said the election was stolen via "unprecedented electoral fraud".

Voters across the country complained over the ease with which the indelible ink was removed from their hands, while videos, pictures and first-hand accounts of purportedly foreign "voters" being confronted by angry citizens also went viral.

A Malaysian government spokesperson said that Wednesday's planned protest "is calculated to create unrest".

While previous election reform protests have ended with police using tear gas and water cannon. Police had earlier threatened to arrest participants in Wednesday night's rally.

But with tension high over the country's closest-ever election result, police backed off and a festive atmosphere prevailed as rally-goers waved opposition party flags and blared vuvuzela horns.