The sentence has been suspended pending an appeal by the five activists in a higher court.
The case refers to a row in May 1998, when Mr Dey, a Right To Information (RTI) activist, had gone to a village near Jaipur to investigate reports of corruption in development projects.
The village head or Sarpanch, Pyarelal, was accused of fudging payments for toilets, homes and other government schemes and cheating actual beneficiaries.
On May 6, Mr Dey and four more activists went to the village to demand details from the Sarpanch in response to the allegations.
The activists allege that when they went to Pyarelal's house to deliver an official letter - finding his office closed - the sarpanch assaulted them and then filed a case against them for trespass and hurt.
The activists did not file any FIR or police complaint, believing, in the early years of the RTI Act, that a dialogue would resolve the dispute more effectively.
The case was closed by the police in 1998, after they inquired into the versions of both sides. It was revived three years later by the Sarpanch, who filed a petition in court and allegedly produced witnesses who were not present during the incident. The verdict was announced yesterday.
"We are deeply disappointed by the verdict," said RTI activist Aruna Roy, calling it a clear case of miscarriage of justice.
"The process of this case that has continued for almost two decades, and the final decision is a body blow to the effort of citizens to fight corruption and stand up for the rights of the citizen," she said.
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