The Bijepur Assembly seat is set to go to polls tomorrow and the counting of votes will take place on February 28. The bypoll was necessitated following the death of Congress MLA Subal Sahu in August last year.
The minister's brother Subrat Singh and his associates were travelling in a car when the miscreants intercepted their vehicle and attacked them around 1.30am in Bargarh district, Additional Superintendent of Police Yashobant Senapati said.
"All four injured persons were rushed to nearby Sohela Hospital and later shifted to Veer Surendra Sai Institute of Medical Sciences and Research (VIMSAR) at Burla for treatment," the officer said.
Three people have so far been arrested in connection with the incident, he said, adding that police have also seized their vehicles.
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"Separate police teams have been pressed into service to arrest more people in the case," he said.
Soon after the incident, the ruling BJD leaders accused the BJP of engineering the attack.
The BJP leaders, however, refuted the allegation as "baseless" and said their party does not believe in violence.
The attack could be fallout of a personal rivalry, BJP spokesperson Sajjan Sharma said.