TAIPING - Malaysia's premier Najib Razak faced a critical test Tuesday, as voters cast ballots in three by-elections seen as a referendum on support for his new leadership.

Najib, who was sworn into power last week, has announced an ambitious agenda to reform the ruling party UMNO, which represents majority Muslim Malays, and repair ties with the nation's ethnic Chinese and Indian minorities.

The headline vote is in Bukit Gantang in northern Perak state, near the former tin mining town of Taiping, where a seat in the national parliament is up for grabs.

The other votes are for seats in state parliaments -- one in the northern state of Kedah and another in Sarawak on Borneo island which until now has been a stronghold of the UMNO-led Barisan Nasional coalition.

"Collectively, these three contests will illustrate whether Najib and his newly elected team have the support of the Malaysian people," said Bridget Welsh, a Southeast Asian specialist at Johns Hopkins University.

Results from the three constituencies, due after 1400 GMT, will provide a snapshot of the public mood, and show whether UMNO has been able to claw back support after a disastrous showing in general elections a year ago.

The three constituencies, embracing more than 98,700 voters, represent a spectrum of Malaysians, including rural Malays who have been UMNO's bedrock, as well as ethnic Chinese and Indians, who shifted to the opposition in the March 2008 elections.

"Given the national representativeness of these constituencies, (whoever) emerges as victor in at least two of these contests will have the potential to secure the majority in the next elections," Welsh said in an analysis.

Political observers say the three-member opposition alliance has a good chance of winning in Bukit Gantang, where Barisan Nasional in January orchestrated the controversial ouster of Perak's opposition state government.

The vote in a remote part of Sarawak is expected to be a tough fight, and the election in Bukit Selambau is complicated by the presence of 13 independent candidates who may leach support from the main parties.

Ismail Saffian, the Barisan Nasional candidate in Bukit Gantang, reiterated the government's cautious hopes for victory.

"Based on my observation I have confidence that I can win. I have met the voters and the response is good. But it's going to be a tough fight," he said as he cast his vote at a schoolhouse polling booth.

Outside the voting centre, dozens of police, including riot squads equipped with tear gas and rifles, stood guard between rival camps of hundreds of supporters who cheered and shouted slogans.

"I always vote for Barisan Nasional, because it is the party that will ensure the future of my children and grandchildren. And I don't want the country to be in chaos," said 74-year-old voter Fatimah Osman.

Among others though there were calls for change, and for the Barisan Nasional to be punished for the ouster of the Perak state government, which was toppled when defections upset the delicate balance of power.

"This government is full of corruption. Perak now has an illegal government," said opposition supporter Sahra Mat Noor, 61.

"This will be a stiff competition, but God willing if there is no cheating, we will win."

The votes are also an important test of whether the opposition, which has won two by-elections in the past year, can continue working together effectively despite wide policy differences between the three members.

In the coastal fishing village of Kuala Sepetang, where almost 50 percent of voters are ethnic Chinese, there was strong support for the opposition candidate who is from the Islamic party PAS.

"I voted for PAS, my four family members also voted for PAS. The opposition is strong here, we want a change," said Beh Lin Huat, a 55-year-old trader.