Mr Williams was alleged to have drawn up a list of targets for firebombings by the militant nationalist group Meibion Glyndwr - Sons of Glendower - said to be behind more than 200 such attacks in Wales.
Defence counsel accused detectives from North Wales Police and MI5 agents of planting incriminating evidence against him. It was claimed they had become frustrated at their inability to secure a conviction.
A second man, Sion Aubrey Roberts, 21, of Langefni, Anglesey, was found guilty of possessing explosive substances, but cleared of conspiring to cause explosions.
The jury has still to agree on a further charge of conspiracy to cause explosions involving Roberts and David Gareth Davies, 33, of Gwalchmai, Anglesey. Members will return to court tomorrow to consider their verdicts. Roberts is also accused of sending incendiary devices to a Welsh Office minister, Sir Wyn Roberts, a Tory agent, Elwyn Jones, and two senior police officers, including the officer leading a special arson squad set up to investigate the fires.
Jubilant scenes inside and outside the court greeted Mr Williams who had spent 15 months in custody. The crowd sang Land of My Fathers in Welsh as he left. During the trial he admitted supporting the burning of holiday homes and claimed that Meibion Glyndwr were regarded as heroes by thousands of young people in Wales. He but denied supporting attacks on people.
Speaking in Welsh, Mr Williams admitted taking part in a rally and march held to commemorate Welsh nationalist 'martyrs' who killed themselves when a bomb they were carrying blew up in 1969.
He said he was a member of 'The Covenanters', a national movement campaigning for Welsh independence by 2000. He said that they were concerned about the impact of English immigration on Welsh communities and the housing market. The movement had been set up after growing disillusionment with the Welsh nationalist party, Plaid Cymru.
Mr Williams denied conspiring with Roberts and said he had only ever spoken to him for a total of 30 minutes. He said that a police officer who has identified him as being a visitor to Roberts' flat 'had made a big mistake'.
The prosecution alleged that Mr Williams was the 'articulate and intelligent' leader of the group. Mr Williams admitted describing politicians as 'gutless collaborators' and described what the English were doing to the Welsh as genocide.
The trial heard how MI5 agents broke into Roberts' flat and planted bugging devices. Security service agents gave evidence hidden behind screens, identified only by letters from the alphabet. One said a bag found inside the flat contained bomb- making equipment, including modified clocks, batteries, wiring, flashbulbs and the contents of shotgun cartridges.Reuse content