Celtic today vowed to "remain strong in the face of intimidation" after it emerged that two viable parcel bombs had been sent to manager Neil Lennon.
Paul McBride QC, who represented Lennon in his recent disciplinary dealings with the Scottish Football Association, and MSP Trish Godman, a Celtic supporter, have also been targeted and Strathclyde Police revealed the packages were "designed to cause real harm to the person who opened them", but all four were intercepted beforehand.
Celtic chief executive Peter Lawwell appealed to supporters to stay calm and only respond through support for Lennon, who has been the subject of tight security measures since another item, initially thought to have been a hoax, was intercepted by Royal Mail staff on March 4.
Lennon last week admitted the off-field issues, which have required him to be put under 24-hour surveillance, had put a "slight strain" on his home life but his love for his job meant he would not give up and Lawwell promised to continue providing support to the Northern Irishman.
Lawwell said: "Neil Lennon is a football manager who simply wants to carry out this role to the best of his professional ability.
"However, in this horrific ongoing campaign, he and other Celtic personnel continue to be the subject of repeated threats and intimidation.
"It is an intolerable state of affairs which must end. Celtic, from our inception, has been a club open to all. We enjoy friendship and respect throughout the world yet, here in Scotland, we are caught up in these vile events.
"The most recent targeting of Neil Lennon, Paul McBride and Trish Godman - three people who are linked only by an affinity with Celtic - deserves condemnation from all right-minded people."
Lawwell appealed to Celtic fans to "respond only through positive support for Neil and the team".
He added: "Celtic and its supporters are more united than ever and we thank them for their expressions of goodwill to Neil at this time.
"Celtic supporters should rest assured that Neil will continue to receive the full backing of everyone at the club. They should also understand that we will remain strong in the face of such intimidation."
Strathclyde Police Detective Superintendent John Mitchell said sending the packages was a "despicable and cowardly act".
Strathclyde Police confirmed they launched a major investigation after two "suspect packages" were intercepted at Royal Mail sorting offices last month, both addressed to the Celtic manager.
The first one was found in Saltcoats, Ayrshire, on March 4 and the second was intercepted on March 26 in Kirkintilloch, East Dunbartonshire.
Two days later, on March 28, a parcel for Ms Godman, the former deputy presiding officer of the Scottish Parliament, was delivered at her constituency office.
The most recent package was intercepted last Friday at a post-box on Montgomerie Terrace in Kilwinning, Ayrshire, and was addressed to Mr McBride.
Rangers assistant manager Ally McCoist said he was "absolutely appalled" at the development.
"I'm really, really saddened that this has taken place in Scottish football," McCoist told BBC Radio Scotland's John Beattie Show.
"We obviously have our differences on the park and that's the beauty of the game, you don't agree with everything that is said and done, but you hope we can conduct these conversations and disagreements in the correct manner."
Lennon received a touchline ban for his infamous disagreement with McCoist following Celtic's Scottish Cup win over Rangers on March 2.
But McCoist insisted they remained friendly away from the game despite the intense rivalry between the clubs.
McCoist, who will take over as Rangers manager in the summer, said: "There is absolutely no way on this earth we hate each other, far from it.
"We have seen each other socially as well as in a working situation.
"We obviously have jobs to do. Our jobs are conflicts of interest obviously, if Neil's team are doing well, our team is not doing well and things like that.
"But at the same time I honestly believe there always has been and will always continue to be a healthy respect for each other.
"Whether it's a Celtic coach, Dundee United coach, Aberdeen coach, there has to be healthy respect for each other, not only between players and coaches but fans as well.
"We always share a bottle of beer or glass of wine after the game and both parties have seen each other socially and there is absolutely no problem at all.
"We were out for example a few Saturday nights ago and it wasn't arranged but the Celtic boys were in as well having a few drinks with their girlfriends and wives, and of course we had a drink and a chat.
"That's what good people do and that's what we believe the Celtic boys are and indeed we are.
"We can't let these idiots win and won't let them win."