They rallied in Sheung Shui, which lies across the border from the mainland city of Shenzhen. For years, traders have bought goods from the district to sell at a markup in Shenzhen. The practice is called "parallel trading" because it happens in a gray area alongside legal trade.
Some black-clad protesters held up signs that read "SARS," in apparent reference to a mysterious infectious disease that may have been brought to Hong Kong from the central Chinese city of Wuhan, where at least 44 people have been infected.
The outbreak has revived memories of the 2002-2003 SARS epidemic that began in southern China and killed more than 700 people.
About 100 protesters marched through a Sheung Shui mall last month, demanding that mainland Chinese traders leave the territory.
Hong Kong, a semi-autonomous Chinese city, has seen more than 6 months of anti-government demonstrations.