Voting got under way at 8:00 am (0000 GMT) as polling centres opened with tensions high after a bitter campaign in the multi-ethnic country marked by charges of election fraud and hundreds of reports of violence.
Malaysians have keenly awaited the vote ever since 2008 polls, when the opposition made unprecedented gains against the once-invincible coalition that has monopolised federal power since independence in 1957.
The coalition dominated by the United Malays National Organisation (UMNO) and led by Prime Minister Najib Razak had been expected to edge the Pakatan Rakyat (People's Pact) alliance captained by charismatic UMNO outcast Anwar Ibrahim.
"It's a tight run. But I'm not scared, I'm excited," said retiree H.Y. Ong, as he waited to vote in the capital Kuala Lumpur.
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"The times have changed, they (the government) need to change. Money politics should be controlled," he added, while not divulging his voting preference.
Pakatan has capitalised on anger over corruption, authoritarianism and controversial policies that favour majority ethnic Malays, while wooing minorities and a younger generation exposed to alternative views found online.
His back to the wall, Najib has offered limited political reforms to stem the tide, but otherwise a largely stay-the-course vision.
The opposition has set the stage for a possibly disputed result with numerous accusations of electoral fraud by the Barisan Nasional (National Front) ruling coalition.
They include an alleged scheme to fly tens of thousands of "dubious" and possibly foreign voters to key constituencies to sway results.
The government claims the flights were part of a voter-turnout drive but has given no other details.