CBI sources said the operation is still going and further details cannot be divulged.
The agency is carrying out surprise checks jointly with Postal Department, they said.
The sources said the action was taken after complaints were received that some officials were allegedly favouring some people in the distribution of currency after Rs 500 and Rs 1,000 notes ceased to be legal tender from November 9.
They said there were complaints of other irregularities too like exchange of currency which are being looked into by the agency.
The sources did not reveal how many more post offices could be part of the check operation as the operation is likely to continue throughout the night.
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During the operation, CBI personnel visited Khairatabad Head Post Office, GPO, Sanathnagar Post Office, and Himayathnagar Post Office to check cash exchange process at these places.
They have not found any irregularity at these POs, K Sudheer Babu, Senior Superintendent of Post Offices in Hyderabad city division, said in a release here tonight.
"It is a routine vigilance visit by CBI at post offices," he added.
