By Hilary Burke LA PLATA, Argentina, Oct 9 (Reuters) - A former police chaplain was sentenced to life imprisonment on Tuesday for involvement in torture, kidnapping and murder during Argentina's "dirty war," in the first trial of a clergyman for rights abuses under military rule. Roman Catholic priest Christian Von Wernich tended to the notorious Buenos Aires provincial police force, and rights lawyers say he pressured torture victims to talk during visits to clandestine detention centers. As the verdict was read in the televised trial, Von Wernich, 69, sat in court with his head hanging low, his mouth turned downward and his arms crossed over a bulletproof vest. He was wearing a priest's collar. Outside the courtroom in the central city of La Plata, crowds cheered and set off firecrackers. Among those in court were the mothers of murdered political activists, wearing the white headscarves that have become a symbol for human rights campaigners worldwide. Thousands of leftist dissidents were systematically maimed and killed during Argentina's 1976-1983 dictatorship. Von Wernich's trial was the first case to probe dirty war-era crimes since a former provincial police commissioner was sentenced to life in prison in September 2006. (Additional reporting by Miguel Lo Bianco and Helen Popper)