The men, aged 41 and 43, were detained this morning after officers raided a number of properties in Kilwinning, Ayrshire. Police later said they had been arrested and detained in police custody under the Explosive Substances Act (1883).

It came as a man appeared in court charged with assault after an attack on Lennon at Tynecastle stadium at the Celtic v Hearts game last night.

Police were also called to Celtic's stadium in Glasgow today after a suspect package which was believed to have contained a bullet and addressed to Lennon was found.

Police launched a major investigation after two bombs were sent to Lennon and one each to lawyer Paul McBride QC and former MSP Trish Godman.

A fifth suspect package, addressed to the offices of Cairde Na H'Eireann (Friends of Ireland) in Glasgow, was also intercepted by officers last month.

All of the parcel bomb packages were found during March and last month.

The two men arrested today are expected to appear at Kilmarnock Sheriff Court tomorrow.

Football chiefs said the recent incidents had brought shame on Scottish football.

Officers arrested a man last night after he clambered from the Hearts section of the main stand on to the pitch during Celtic's 3-0 win at Tynecastle and made towards Lennon.

The manager, who has had to live with round-the-clock security after death threats in the past, was said to have been left "shaken" by the incident.

John Wilson, 26, from Edinburgh, has been charged with breach of the peace aggravated by religious prejudice and assault aggravated by religious prejudice.

He appeared at a private hearing at Edinburgh Sheriff Court this afternoon. No plea or declaration was made during the hearing and he was remanded in custody.

After last night's incident, trouble flared in the Celtic section of the stadium, with fans appearing to fight with police and stewards.

Police stressed that the suspicious package discovered at Celtic this morning is not being linked to the parcel bombs inquiry at this time.

They said the investigation is at an early stage and it has not yet been confirmed where it was sent from.

Hearts and the Scottish Premier League (SPL) have also launched investigations into what happened at Tynecastle.

SPL chief executive Neil Doncaster told BBC Radio Scotland the incident was "wholly unacceptable" and "disgraceful".

Scotland's First Minister Alex Salmond branded such behaviour "utterly unacceptable".

Earlier this week, seven people appeared in court charged with possessing an imitation firearm after they were arrested outside Celtic's training ground.

Celtic chief executive Peter Lawwell said: "It is intolerable that any football club, or individual, going about their lawful business in the name of sport should be subjected to this ongoing campaign of hatred and intimidation.

"This is Scotland's shame and it is high time Scotland addressed it."

He said Lennon deserved respect for the strength of character and resilience he has shown during the "campaign of intimidation" he has endured.

He added: "He is a man who is proud to be the Celtic manager and is someone who simply wants to be able to carry out his role in the same way as every other football manager can. Clearly, Neil will continue to receive every support and protection from Celtic Football Club."

Hearts have banned the supporter involved in last night's incident for life.

A club spokesman said: "While not wanting to prejudge the outcome of the court case, all true Hearts fans will have been disgusted by what they witnessed last night.

"There is no justification for such actions, which have no place in the game of football, and we have taken the severest action open to set the strongest possible deterrent.

"As such, the supporter involved has been handed a life ban from Tynecastle Stadium."