The dismissal last month of the Ming Pao newspaper's No. 2 editor, Keung Kwok-yuen, came after the newspaper published a report on the "Panama Papers" document leak revealing offshore business dealings of the rich and powerful.
The newspaper said Keung was dismissed to save operating costs.
The South China Morning Post said journalists, activists and politicians attended the rally outside the Ming Pao Industrial Center. Protesters carried signs saying "protect journalists, protect Ming Pao, protect press freedom." The chairwoman of the Hong Kong Journalists Association, Shan Yee-lan, read a joint letter from eight journalist groups calling for Keung to be reinstated, the newspaper said.
Before Keung's dismissal, Ming Pao carried a front-page report on Hong Kong politicians and businesspeople named in documents leaked from a Panama law firm and published by the International Consortium of Investigative Journalists.
There has been growing anxiety in recent years among many Hong Kong journalists and politicians about the influence of Beijing on the territory, which retained its own civil liberties when handed over from Britain in 1997.
Media groups with close business and personal ties to Beijing have been accused of soft-pedaling their coverage of issues that are potentially embarrassing to China and its allies in Hong Kong.