Sam Rainsy said the large influx of Chinese immigrants to Cambodia, coinciding with a construction boom and waning relations with the West, could lead to a backlash from locals.
Beijing and Washington bicker over history in Cambodia, blaming each other for the rise of the brutal Khmer Rouge regime.
Cambodia also urged the international community not to interfere in its internal affairs and respect its sovereignty.
Sam Rainsy left Cambodia in 2016 facing a number of lawsuits filed by senior officials.
Long Beach is the second-busiest port in the United States after Los Angeles and is home to more than 20,000 Cambodian-Americans.
As trade war goes on, China joins WTO talks on e-commerce but conflicts expected
Democrats fear independent candidacy could help reelect President Trump
U.S. Vice President Mike Pence expressed concern over the report in a letter to Prime Minister Hun Sen, whose government denied the allegation.
Army chief Apirat Kongsompong arrived on Tuesday and met with Cambodian army commander Hun Manet and Defense Minister Tea Banh.
The bill aims to apply travel and economic sanctions on individuals who have “undermined democracy” in Cambodia and violated human rights.
Sam Rainsy says he will return to Cambodia some time this year from France, where he has lived since fleeing in 2015 after conviction for defamation.
Populist forces returned with a vengeance to Europe in 2018, seizing power in Italy and extending their grip in countries like Hungary and Poland. VOA Khmer's Phy Sopheada narrates.