Port Authority of Thailand (PAT) employees are extorting shipping operators at Bangkok Port, the Thai Federation on Logistics claims.
The shipping operators will front the media today to provide details of the alleged corruption, Thongyu Khongkhan, the federation's chief adviser and executive, said.
The port's union, meanwhile, says it will press ahead with its campaign for the PAT director-general to quit.
About 300 employees belonging to the PAT union rallied on Friday at the port's headquarters in Klong Toey district, disrupting container movements.
The union is trying to pressure the board to remove PAT director-general Viroj Chongchansittho for his alleged failure to compromise in an overtime dispute.
The protesting workers stopped working overtime on Friday, which delayed shipments from the port.
The workers resumed their duties on Saturday night following talks with PAT chairman Pol Lt Gen Khamronwit Thoopkrachang, who is also the city police chief. However, staff threatened to resume their protest if the board, whose committee will meet today, fails to heed on their demands.
Mr Thongyu said the federation and shipping operators had evidence to back their allegations against PAT employees.
He said shipping operators have been forced to pay bribes to PAT employees responsible for controlling the machines that lift cargo containers.
Operators had to pay an extra 50 baht each to have an empty container lifted, and an extra 100 baht for a full container.
The containers would not be lifted unless the extra money was paid, Mr Thongyu said.
He also said PAT employees were claiming overtime for hours they did not work.He vowed to expose what he called systematic corruption at the state agency.
Chalongrat Kulsanan, deputy chairman of the PAT labour union, said the union may ask employees to stop working overtime again or take leave unless the PAT director-general is removed.