Vowing to "never surrender", opposition leader Anwar Ibrahim called on Malaysians Wednesday to join in a nationwide protest tour against elections he said were stolen from the country's people.
Anwar addressed a sea of supporters dressed in mourning black who filled a football stadium and spilled out into surrounding areas, swamping a suburb of the capital Kuala Lumpur in a gathering with a rock-concert atmosphere.
"We will go to every corner of this country," Anwar declared, prompting roars from the crowd.
"We will continue to struggle and we will never surrender!"
The huge turnout and Anwar's call for similar rallies across the country upped the ante in a campaign by the opposition to paint the elections as a fraudulent victory for the regime that has ruled Malaysia for 56 years.
Anwar had already vowed a "fierce" campaign against Sunday's poll result and said he would soon produce evidence of fraud by what he calls an "illegitimate" Barisan Nasional (National Front) government headed by premier Najib Razak.
"BN has robbed the rights of the people. We will prove that they have lied in 30 parliamentary seats," he told the ecstatic, multi-racial crowd.
Najib's government has hotly denied the opposition's allegations of cheating.
It had earlier denounced the gathering in the 25,000-seat stadium, which was filled to its seating capacity and had at least twice that many on the football pitch. Thousands more were outside.
Najib's office had issued a statement before the rally saying it was "calculated to create unrest".
But the crowd was more festive than angry, roaring for a succession of opposition leaders and clean-election activists as rally-goers waved opposition party flags and sounded vuvuzela horns.
"I think they should re-do 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."
Previous election-reform protests have ended in wild scenes, with police using tear gas and water cannon.
Police had earlier threatened to arrest participants at Wednesday's rally but with tension high over the country's closest-ever election result, they backed off and little security presence was seen.
Anwar has battled Barisan since he was ousted from its top ranks in 1998 and jailed for six years on sex and corruption charges widely seen as trumped-up.
The opposition leader, who had urged Malaysians across the country to wear black in protest, called for another rally in his northern home state of Penang on Saturday, with more to follow around Malaysia.
Among other allegations, voters complained that indelible ink -- meant to thwart multiple voting -- easily washed off. Accounts of suspected foreign "voters" being confronted by angry citizens at polling centres also went viral online.
Anwar had earlier alleged a government scheme to fly tens of thousands of "dubious" and possibly foreign voters to flood key constituencies.
The government has poured scorn on the allegations.
But a report released Wednesday by two independent watchdogs said the polls were marred by pro-government bias and irregularities that indicate "serious flaws" in the electoral system.
The Institute for Democracy and Economic Affairs and Centre for Public Policy Studies cited concerns including partisan use of government machinery, pro-government media bias and doubts over the integrity of voter rolls.
The election was "only partially free and not fair", the report said.
The vote was touted as the first in which the opposition had a chance to unseat the ruling coalition, which has governed since independence in 1957.
Barisan retained a firm parliamentary majority despite winning less than half the popular vote, a factor blamed on gerrymandering and Barisan tinkering with electoral districts, and adding to opposition supporters' anger.
Both the United States and European Union congratulated Najib on his win but urged him to address reports of irregularities, while anti-graft watchdog Transparency International said the vote showed electoral reforms were "urgently required".
The White House Wednesday added its voice, saying it was important that Malaysia addresses "concerns that have been raised".
Suspicion over the result could cause an extended period of unrest, according to Joshua Kurlantzick, a Southeast Asia Fellow with the Council on Foreign Relations in Washington.