A man falls onto the ground during a confrontation between protesters demonstrating against mainland traders and local residents who feel the protests are disrupting their business, at Yuen Long district near the border with mainland China in Hong Kong March 1, 2015. [Photo/Agencies]

Violence against parallel traders and mainland visitors in Hong Kong has triggered widespread concern among academics and even the most radical faction in the opposition camp.

As of Thursday, at least seven people had been arrested for alleged violence related to anti-parallel trading protests staged on Sunday.

News footage showed an elderly man being bullied by a group of masked activists, and a young girl bursting into tears as her mother quarreled with protesters.

Both the man and the mother had been pulling luggage near a protest rally. The man was later identified in media reports as a local resident who regularly plays music at a public park in the satellite town of Tuen Mun.

Parallel trading in Hong Kong refers to the phenomenon of mainland and Hong Kong traders taking goods from the city to the mainland, causing shortages of household goods in various locations in Hong Kong.

Protests against daily visitors from the mainland have swept northern areas of Hong Kong since early last month.

The first, a march, began in Tuen Mun on Feb 8 as an approved rally, but crowds ended up storming a shopping mall. The protests later spread to the towns of Yuen Long and Sha Tin.

Radical lawmakers from Hong Kong's opposition camp, known for their filibusters in the Legislative Council that have stalled funding requests, on Wednesday stood together against Sunday's incidents, saying that violence is unacceptable.

Lau Siu-kai, vice-president of the Chinese Association of Hong Kong and Macao Studies, said the opposition camp has drawn a clear line with the radical groups, which should help mounting calls for law enforcers to take stern action.

Dennis Wong Sing-wing, a sociology expert at City University of Hong Kong, said netizens have been elated by recent media exposure, which they had struggled to achieve previously.

Chung Kim-wah, a policy expert from Hong Kong Polytechnic University, said the protesters may feel they have received a further boost by the authorities' move to revise the individual visit scheme for mainland residents.