They have also demanded withdrawal of Faridpur SP and Madhukhali Police Station OC.
The threat to convert their strike into a national shutdown was made at a press conference at Jessore Bus Owners Association office on Saturday.
Khulna Division Owners Workers United Council Convener Ali Akbar read out the written statement.
He gave an ultimatum of three days to meet their demands.
“There is no chance to backtrack from this protest. Discussions with the owners and workers of all divisions, including Rajshahi, Barisal and Dhaka are underway,” Akbar said.
Police detained the driver of the Sohag Paribahan bus and his assistant for interrogation after the robbery at Faridpur’s Madhukhali on May 18.
The detained driver and his assistant were later shown arrested in the robbery case.
But their fellow workers and transport owners claimed police arrested them when the driver went to Madhukhali Police Station to file a complaint.
On last Wednesday, they called the strike in the southern districts demanding unconditional release of the two and withdrawal of the Faridpur superintendent of police and Madhukhali Police Station OC.
No buses from Faridpur, Jessore, Kushtia, Jhenaidah, Khulna and Satkhira travelled on the roads leading to Dhaka until Saturday. Intra-district routes too were closed.