The relatively developed country is a magnet for migrant workers from across Asia, but several million are believed to be undocumented.
Authorities started rounding up illegal workers after an official programme to register undocumented foreigners ended on June 30, and more than 3,100 had been detained as of July 11, according to the immigration department.
Bangladeshis represent the largest group among those detained, with substantial numbers also from Indonesia and Myanmar. In the latest raid late Tuesday, scores of foreign workers at a construction site in the coastal town of Port Dickson were rounded up.
The clampdown has caused alarm among activists and in neighbouring countries, and a group representing current and former lawmakers from across Southeast Asia added their voice to the concern.
"A desire to decrease the number of undocumented workers in the country can never be an excuse to further victimise the vulnerable," said Cambodian lawmaker Mu Sochua, a board member of the ASEAN Parliamentarians for Human Rights.
"While Malaysia has a legitimate need to address the fact that so many migrants find themselves without proper paperwork, it must ensure that basic human rights are respected for all people at all times."
Aegile Fernandez, director of prominent Malaysian migrant rights group Tenaganita, said the real culprits were unscrupulous agents who extorted money from foreign workers in exchange for bringing them to the country, leaving them saddled with huge debts.
"Malaysia would not have developed so much if not for migrant workers. We should thank them and not handcuff them," she told AFP.
More than 60 employers who allegedly hired illegal foreign workers have also been arrested. Migrant workers typically do jobs spurned by locals, such as on construction sites and palm oil plantations.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)