In the raids that began on Tuesday, immigration officials armed with pistols stormed workers' huts, rousing 243 labourers from their sleep and finding 19 without proper papers.
"We will detain them for 14 days and check out their documents," said Mahadi Arshad, the chief of a volunteer force that is spearheading the campaign to drive out illegal immigrants, most of whom come from poorer neighbour Indonesia.
"Then immigration authorities will decide whether to deport or to detain them," he added.
Similar scenes played out across the country in the first hours of Malaysia's biggest crackdown on illegal immigrants since 2002.
The question of illegal labour is controversial in Malaysia, where a government drive to improve education and modernise the economy means fewer people are willing to do some of the gruelling manual work upon which the country's prosperity has been built.
The roundup follows a four-month amnesty, which ended at midnight on Monday, under which about 400,000 illegal workers have left the country without punishment, the authorities say.
"Immigration authorities will decide whether to deport or to detain them"
Mahadi Arshad, chief of volunteer force
The government says they will be allowed to return under a revolving door policy once their paperwork is in order.
But an estimated 200,000 to 400,000 remain, willing to run the risk of a fine and jail or caning for men younger than 50.
The authorities have said migrants detained after the deadline will be barred from returning to Malaysia while those who left voluntarily under the amnesty have been offered the chance to re-enter through legal channels.
Malaysia suffers a chronic shortage of labour and relies heavily on low-wage workers from Indonesia, just a ferry-boat ride away, to take up unskilled or semi-skilled work at construction sites, factories, plantations and restaurants.
But the government is concerned these workers do not pay tax and put a heavy burden on the state, which runs a budget deficit.
Malaysia relies on labour
from neighbouring Indonesia
The volunteer Rela force - a neighbourhood watch organisation - says illegal workers are responsible for crime and other social ills.
Mahadi said about 25,000 of the 300,000 members of his force had been trained and deployed to flush out illegal workers.
Led by six armed officers, about 400 volunteers marched into the construction site at Cheras, 8km from the centre of Kuala Lumpur, and trudged through mud to reach the workers' huts, woke up the labourers, and lined them up in rows.
Raids to hunt illegal migrants were also being carried out in Malaysia's eastern states of Sabah and Sarawak, Mahadi said, but he had no results of those operations yet.
The roundup promises to be Malaysia's largest such operation since 2002, when human rights groups criticised the nation for holding people in crowded detention centres and deporting them en masse, including some likely to be genuine asylum-seekers.
At times, Malaysia's illegal immigrant problem has been almost comical.
Thousands of Indonesians surrender themselves for deportation each year before the Muslim holiday of Eid.
Given a free passage home, they enjoy a break with their families before sneaking back across the border to resume work.