Workers' Party chief Low Thia Khiang on Thursday accused the PAP Government of kicking the can of economic restructuring down the road and using immigration as the easy way out.
He called on it to stop using this policy tool - which he said is the cause of problems today - as the solution for tomorrow.
Instead, he said he believes the WP's proposal of raising the labour force participation rate among Singaporean women and seniors, is more sustainable.
The party has also said it does not want to take in more foreigners than there are now.
He spoke in English and Mandarin to object to the Government's White Paper on Population and Land Use Plan.
Mr Low also argued that the Singaporean core will be diluted and that it would be hard to ensure integration of new citizens with "thorough-bred Singaporeans", whose ancestors came from overseas but whose national identity was formed over decades of nation building.
He also accused the Government of downplaying the 6.9 million population as a "worst-case scenario" and wanted it to clarify if it has control over the inflow of foreigners or that Singaporeans should be prepared for an impending immigration tsunami.
Describing the WP as the "responsible co-driver", Mr Low also said it is now telling the driver that "he is reading his road map upside down".