The threats have been roundly condemned in Scotland.
Here is what some political and footballing figures have said.
ALEX SALMOND, FIRST MINISTERThere is a major police investigation under way to ensure that the individual or individuals concerned are identified and apprehended, and then brought to book with the full force of the law.
These disgraceful events should remind all of us who love the game of football of what unites us as a community.
It is time to remember what we value in society, and unite to condemn those who use football as a pretext for their pathetic and dangerous prejudices.
IAIN GRAY, SCOTTISH LABOUR LEADERI am absolutely appalled at this development and hope that progress can be made on finding those responsible.
Individuals who engage in this type of terrorism do not have the support of the public and are rightly seen as pariahs.
Now is the time for calm heads. I hope that the whole community shows that this type of behaviour will not be tolerated in our country.
ALLY MCCOIST, RANGERS MANAGERLike any other normal human being, I'm absolutely appalled that in this day and age... people should see in some bizarre and crazy and outrageous way that they've got to send these devices.
Unfortunately I think the sad thing is, no matter what I say or do, or what other people say or do, I don't think it's going to have a major effect on these people.
Because, to be quite frank, they're sick. We cannot let these idiots win. And we won't let them win.
STEWART REGAN, SCOTTISH FOOTBALL ASSOCIATION CHIEF EXECUTIVEThe Scottish FA is horrified and saddened by the news that the Celtic manager, Neil Lennon, has been subjected to another - reportedly 'viable' - threat along with two other prominent supporters of the club.
It is hoped that swift action by Strathclyde Police will succeed in capturing the perpetrators of these cowardly acts and bring the full weight of justice down on them.
Scottish football should be a safe and entertaining environment for players, coaches and supporters. It must not be used as a platform for religious intolerance or hatred.
MICHAEL KELLY, FORMER CELTIC DIRECTORThis now is terrorism, purely and simply. It's got nothing to do with football and the background of the summit and the Old firm game.
It's up to the police to refocus their targets on these people and to catch them.