The race for the mainly rural seat located about 44 miles (70km) from the capital pits opposition leader Anwar Ibrahim's party against the National Front coalition.
Malaysia's opposition put questions over prime minister Najib Razak's ability to deliver on his reform pledges at the centre of a tense by-election campaign which started on Saturday.
The race for the mainly rural seat located about 44 miles (70km) from the capital pits opposition leader Anwar Ibrahim's party against the National Front coalition and could provide an early verdict of Najib's reforms.
Thousands of opposition supporters chanted "down with Najib," and "long live the people" as they gathered opposite an equally large number of government supporters at the nomination centre in Hulu Selangor on Saturday morning.
Political tensions in this Southeast Asian country have been high since a general election in 2008, in which the coalition that has now ruled Malaysia for 52 years suffered record losses.
Hundreds of riot police kept watch on the rival groups of supporters who were separated by barbed wire as police helicopters circled overhead in the constituency the opposition won in the 2008 general election for the first time with a majority of 198 votes.
"Najib was someone who wanted to be everything to all people and ended up finally satisfying almost no one," said Dzulkifli Ahmad, a senior figure in the Pan Malaysian Islamic Party (PAS), one of the three parties in the opposition People''s Alliance.
Najib, who took office in April last year, has pledged to woo foreign investment and revive his ailing coalition. Last month, he announced a series of economic reforms to boost growth.
Meant to replace Malaysia''s four-decade-old race-based affirmative action policies, the new plans have triggered fears of a backlash from the majority ethnic Malay population.
Conservative rights groups have sprung up demanding that privileges be preserved for Malays, who make up about 55 percent of the 28 million population and are the core voter base for Najib''s own political party.
Analysts worry the moves to pander to the Malay vote-bank may end up pressuring Najib to water down reforms.
The stakes are high for the opposition as well, which is fielding 59-year-old former law minister Zaid Ibrahim.
Zaid is seen as one of opposition leader Anwar Ibrahim''s potential successors should Anwar be convicted of sodomy in a trial set to end in late August.
The by-election will be one in a series of "warm-up" polls ahead of the next general election, which is not due until 2013 but which could be held as early as next year.
Political uncertainty in Malaysia since the 2008 elections has helped dent foreign investment -- net portfolio and direct investment outflows reached $61 billion in 2008 and 2009, according to official data.
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