The People's Democratic Reform Committee (PDRC), led by Luang Phor Buddha Issara, marched from Muang to other districts in the province urging people to join its rally to seize Bangkok on Monday.
The demonstrators encountered opposition from United Front for Democracy against Dictatorship supporters led by Wutthipong Kochathammakhun in Lam Luk Ka district, said Pol Maj Gen Smit Mukdasanit, chief of the Pathum Thani police, said.
They clashed and four people were injured, one seriously, after gunshots and giant firecrackers were heard, he added.
Khu Khot police said two of the injurwed were believed to be red-shirt members and another one was a student walking in the area.
PDRC member Satit Wongnongtoey called on the government to take responsibility for the clash
Earlier, Mr Wutthipong had led the same red-shirt onvergegroup to the offices of Daily News and Thai Rath newspapers to submit a letter complaining about their news coverage.
The letter demanded the media support the Feb 2 election and provide more balanced news coverage, a Daily News editor said.