Politicians said the attempted bombings showed action was needed to deal with religious bigotry and sectarian violence between some fans of the club and its city rivals Rangers, which has flared anew in recent months.
Michel Platini, the president of European football's governing body UEFA condemned the action while on a visit to London.
Platini spoke to reporters after handing over the European Cup trophy to London mayor Boris Johnson for this year's Champions League final which is at Wembley Stadium next month.
"I am with him (Lennon) in my heart. I think we must keep politics and religion out of football. There will be an inquiry and what has happened to the manager of Celtic is very bad.
"It is not permitted. Everyone in football supports him. We will fight against violence, sectarianism, everything. We will fight, but if someone is sending a bomb to someone else, that is very complicated for UEFA to manage that."
Lennon, a Northern Irish Catholic, who has been the subject of several threats and physical abuse in the past, has himself been accused of helping to stoke the tensions, along with senior figures at Rangers.
"Sending these types of packages through the post is a despicable and cowardly act," Detective Superintendent John Mitchell of Strathclyde Police told reporters.
"I can confirm that they were designed to cause real harm to the person that opened them."
While commentators have suggested an individual was probably to blame, police have not yet ruled out the involvement of a guerrilla group. Dissidents opposed to the Irish peace process have stepped up activities in recent weeks.
BITTER RIVALRY
The acrimony between Celtic, which has a predominantly Catholic fan base and Rangers, whose supporters are mainly Protestant, is long-running and deep-seated.
The sectarian divide echoes the religious and social hatred that has plagued Northern Ireland; Celtic's fans regularly wave Irish flags while Rangers' supporters brandish British and Northern Ireland flags on match days.
The two clubs, known as the "Old Firm" meet again on Sunday in a potentially decisive Scottish Premier League match.
Lennon withdrew from the Northern Ireland team when a player in 2002 after a death threat which media speculated had come from Protestant paramilitaries in the province.
In January, packages, posted in Northern Ireland, containing bullets were sent to Lennon and a Celtic player.
Police said the first suspect parcel bomb, addressed to Lennon, was intercepted by Royal Mail staff on March 4.
Further packages were sent to the Celtic manager, to the office of former deputy presiding officer of the Scottish parliament Trish Godman and to Lennon's lawyer Paul McBride, with the most recent intercepted by mail staff on April 15.
The bombs were initially thought to be hoaxes but police said forensic tests had now shown they were viable. Detectives declined to giv
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