Sri Lanka has been trying to woo investors after defeating Tamil Tiger rebels in 2009 and declaring an end to nearly four decades of ethnic violence that killed up to 100,000 people.
Sevanagala Sugar Industries, which is controlled by an opposition politician, said that ruling party activists had forcibly evicted the management from the sugar plant in the south of the island on Thursday.
The government, which declined to comment on the raid, has defended the proposed law to nationalise "under-performing" private companies and it rejected allegations the policy was designed to crush political opposition.
President Mahinda Rajapakse's administration is set to introduce the bill next week in parliament where his party commands a two-thirds majority.
Rajapakse, who exerts huge control over Sri Lankan politics, has been accused by international rights organisations of undermining media freedom and thwarting dissent, charges vehemently denied by the government.
The president is al