Gerrah Selby, 20, Daniel Wadham, 21, Gavin Medd-Hall, aged 45, and 41-year-old Heather Nicholson orchestrated the campaign against companies who supplied Huntingdon Life Sciences.
All denied the charges at Winchester Crown Court. A fifth defendant, Trevor Holmes, 51, was cleared.
Three other people - Gregg Avery, Natasha Avery and Daniel Amos - pleaded guilty to conspiracy to blackmail at the beginning of the trial.
The group targeted hundreds of businesses with links to Huntingdon Life Sciences, from small haulage firms to the international pharmaceutical giant GlaxoSmithKline in an attempt to stop the testing laboratory being able to operate.
Their campaign aimed to create a "climate of fear" not only for firms in the UK, but also those in Europe and Japan.
They sent hoax bombs, vandalised cars and property and posted sanitary towels allegedly contaminated with .
They also used the internet to publish details of people working for the targeted companies, including whether they had children and how easy it would be to access their property.
William Denison, who runs a chemicals company in Preston linked to HLS, was the subject of a vicious campaign in which his neighbours were sent letters telling them he was a paedophile with serious child abuse convictions in Thailand.
A letter sent to the Managing Director of Lancer UK, a firm which supplies decontamination washing machines to HLS, warned: "We will attack your property, your family or you, whichever we see fit...the screams of our animals are in our heads. We will not fail them."
Other letters contained chilling warnings that the bodies of dead relatives would be dug up.
This was seen as a reference to the case four years ago, in which animal rights extremists exhumed the remains of Gladys Hammond in Staffordshire after her family's guinea-pig farm was linked to Huntingdon Life Sciences.
Detective Chief Inspector John O'Connor, from Kent Police, said: "This was a very important case. For the victims this was a significant threat to their way of life. They were living under the fear of what may happen.
"This was a very large operation. The consequences have been dramatic. When the extremist element is being dealt with in this way it does create the opportunity for voices from both sides of the animal research debate to come forward and speak up. "
But while the police are hailing these convictions as a victory, some within the animal rights movement insist this case will do nothing to diminish their resolve.
Keith Mann has been a leading figure in the Animal Liberation Front for many years, and spent most of the 90s in prison for his own letter-bomb campaign.
Speaking exclusively to Sky News, he poured scorn on the police claim that the convictions had taken the heart out of the movement.
"If the heart's gone, if we're ineffective any more, why are they putting so many resources into containing us and controlling what we're doing?" he said.
"Very recently in 's speech they announced new legislation giving yet more powers to the police to deal with the so-called threat of animal rights activists. If the threat is gone, why do we need more laws to contain this so-called threat?"
The arrests that led to the trial at Winchester were made during Operation Achilles last year.
It was the UK's largest-ever police crackdown on animal rights extremists. It involved more than 700 officers from five separate forces and was spearheaded by the National Extremism Tactical Co-ordination Unit.
Sentencing will take place on January 19.
