The April 7 balloting is primarily a contest between the National Front ruling coalition and opposition leader Anwar Ibrahim's three-party People's Alliance, which made major inroads in general elections a year ago.
The seats - one in Parliament and two in state legislatures - became vacant recently after two incumbents died and one resigned.
Candidates had their petitions to run for office approved Sunday by the Election Commission.
The elections will not change the balance of power in any of the three bodies. But they will test the leadership and organizational skills of Deputy Prime Minister Najib Razak, who is scheduled to become prime minister within a week, and newly elected leaders of the United Malays National Organization ruling party.
The party's new deputy leader, International Trade Minister Muhyiddin Yassin, led some 10,000 government supporters at Taiping town, in Perak state, to back their candidate, who filed his papers to contest the Parliament seat.
However, they were outnumbered by about 20,000 opposition loyalists, many of whom were upset at tactics used by the National Front to wrest control of Perak from the opposition earlier this year. The Front coaxed three lawmakers to leave the opposition, tipping the balance of the state legislature in its favor.
'We believe the opposition will win (in Perak) because of the public's anger, but we cannot be so sure about the other two seats,' said opposition supporter Ismail Samsuddin, who was selling umbrellas decorated with political emblems.
The National Front's popularity has plunged to its lowest levels in 52 years of rule. It retained power with less than its longtime two-thirds parliamentary majority in national polls a year ago and lost two subsequent by-elections.