Angie, the officer's widow, was flanked by her sons Louie, seven, and Joshua, three, as she led mourners at the service at St Columbus Church in Lisburn, Co Antrim.
Reserve Constable Johnston and his colleague, John Graham, were shot in Lurgan, Co Armagh, on Monday. A man arrested in connection with the killings remains in police custody.
A private service for the family and friends of PC Graham was held yesterday in his home town of Richill, Co Armagh.
Ronnie Flanagan, the RUC Chief Constable, and members of the force, joined friends and relatives of PC Johnston to hear the Presbyterian Moderator, Dr Sam Hutchinson, call on the Government to turn its back on attempts to bring the men of violence in from the cold.
He told the hundreds of mourners crammed into the church, and the many more standing outside: "With these murders Northern Ireland has passed a watershed, one of those critical points after which things can never be the same again. These killings were a slap in the face for so many people of goodwill who were trying to promote understanding and take risks for peace."
He said there were many who had doubts from the beginning "whether much would be achieved by talking to Sinn Fein. But they were honourable people who were willing to try, lest any chance for peace might be missed".
The policeman's sons paid their own tribute. Louie wrote: "I wish this had never happened to you. I wish it could be someone else. I am sorry that had happened to you. Greatest dad in the world. We love you."
Photograph Brian Little/PAReuse content