The blast occurred about 11:30 a.m. as the workers were cleaning the median strip of a street near Pom Mahakan Fort, the Bangkok Post reported.
Two of the six injured workers were in serious condition while the other four received minor wounds, the newspaper said.
The explosion occurred where members of the People's Democratic Force to Overthrow Thaksinism had been encamped.
Teacher apologizes for showing sexual image of herself in class
Jessica Simpson shares three-way kiss with friends in photo