Two delivery workers have tested positive for COVID-19 after attending a mass rally in protest of their working conditions earlier this week, officials said Friday.

The two were among some 4,000 members of the Parcel Delivery Workers' Solidarity Union who staged a rally in Seoul on Tuesday and Wednesday to coincide with negotiations between the union and logistics firms over implementing an overwork prevention agreement.

Both work for the Korea Post, the national postal service.

"According to the test results we have so far, two people who work at the same place have tested positive, while everyone else there has tested negative," the union said in a statement.

It said that all participants in the rally were asked to enter their names in a log, check their temperatures and sanitize their hands, while any business with workers showing symptoms of a high fever was asked not to participate.

COVID-19 tests are currently under way for all participants, it added.

"We will disclose the results of the COVID-19 tests in a transparent manner as soon as they are aggregated and sincerely cooperate with the health authorities."

The Seoul metropolitan government reported the union to the police on Wednesday for holding the rally in defiance of a COVID-19 ban.

Police have begun investigating the rally's organizers on possible charges of staging an illegal rally. (Yonhap)