Strathclyde Police said the two packages sent to Lennon, and others to lawyer Paul McBride and former MSP Trish Godman, were “designed to cause real harm to the person who opened them”.
The move signals a dramatic escalation of football tensions in the divided city of Glasgow, police said.
The packages, which reports said contained liquid-based bombs, were originally thought to be hoax devices but police in Scotland’s biggest city discovered they were viable after conducting tests.
Scotland’s First Minister Alex Salmond said the explosives were powerful enough to “maim or even kill” and warned the bombs represented a worrying new development in the football-related violence that has long plagued Glasgow.
“That does take it to a new level and that is why we are treating it with all seriousness,” he told BBC television.
“I am sure everyone in Scotland will be fully behind the police in apprehending and dealing with the lunatic element who are responsible for this attempted outrage.”
Detective Superintendent John Mitchell, of Glasgow’s Strathclyde Police, said: “They were definitely capable of causing significant harm and injury to individuals if they had opened them.”
Other high-profile figures with links to Celtic were being given security advice, police said.
The upsurge in long-running sectarian tensions between Celtic, predominantly supported by Catholics, and rival Glasgow club Rangers, whose followers are mainly Protestants, comes after recent trouble at Scottish football games.
Sectarian violence related to the clubs, known collectively as the Old Firm, has always been present in Glasgow but the situation has deteriorated markedly this season.
An ill-tempered Scottish Cup clash last month saw more than 200 arrests inside and outside the stadium and a heated touchline confrontation between Lennon and Rangers assistant coach Ally McCoist.
The red-headed Lennon, a Catholic from the North, has been the target of sectarian attacks throughout his career, including an assault which led to two men being jailed in 2009.
Lennon has also had bullets sent to him in the post.
The 39-year-old retired from international football in 2002 after receiving death threats, widely believed to have been made by extremists opposed to a united Ireland.
Scottish Football Association boss Stewart Regan said he was “horrified and saddened” by the attempted bomb attack.
“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.”
The package addressed to Lennon was discovered at a post office on March 26 in the town of Kirkintilloch, just north of Glasgow.
Labour party politician Godman’s parcel was delivered on March 28 to her constituency office in Bridge of Weir, 24 kilometres west of Glasgow, while the package for McBride was intercepted on April 15 en route to law offices in Edinburgh.
McBride is one of Scotland’s highest profile lawyers and has represented Lennon in disputes with the Scottish Football Association.
Mitchell appealed for help tracking down the perpetrators:
“It is important to say that there is no doubt that there is someone out there with information that can assist us and take this inquiry forward, and the quicker the better.”
THE parcel bombs sent to Celtic manager Neil Lennon are not the first time he has been targeted by off-the-pitch hatred during his football career.
The 39-year-old was born in Lurgan, Co Armagh, and grew up supporting Celtic. He moved to the club in December 2000, becoming captain in 2005 and then manager last year.
He was capped 40 times by Northern Ireland and in the first game for his country after signing for Celtic was jeered by some sections of the home support during a friendly against Norway in Belfast.
In his 2006 autobiography, Neil Lennon: Man And Bhoy, the Celtic manager said he expected some “stick” for being a Celtic player playing for Northern Ireland but nothing prepared him for “the sheer scale of what happened before and during that match”.
A few days before the game, the words “Neil Lennon RIP” appeared on a wall in Lisburn.
Lennon retired from international football in August 2002 on receiving a death threat, reportedly from a loyalist paramilitary group, before he was due to captain Northern Ireland in a match against Cyprus.
In September 2008 he was treated in hospital after being attacked in Glasgow’s Ashton Lane, hours after the opening Old Firm game of the season in which Lennon was the Celtic coach.