Nine people were acquitted in the case.
Prosecution lawyers and police said a manhunt has been launched to arrest the absconding convicts.
Kalidas Baral, who was a lawyer by profession, was shot dead on August 20, 2000. He was the President of the District Puja Udjapon Parishad, central leader of Hindu, Buddhist-Christian Oikya Parishad and local leader of ruling Awami League.
"He was a victim of political rivalry...the rivals within the party had decided to kill him as they could not face him politically," a relative earlier said.
But his relatives said they were unhappy with the verdict. Baral's wife said she would appeal in a higher court against the judgment as more culprits remained beyond the justice.