Two bombs were sent to Lennon and one each to lawyer Paul McBride QC and former MSP Trish Godman.
The men, aged 41 and 43, were detained under the Explosives Substances Act 1883 after officers raided a number of properties in Kilwinning, Ayrshire.
The police operation was launched at around 6am today.
It follows police stopping and questioning motorists and pedestrians in Saltcoats and Kilwinning on Friday as part of their investigation.
A package sent to Lennon on March 4 was traced to a postbox in Gladstone Road, Saltcoats, Ayrshire, while one addressed to Mr McBride was found at a postbox in Montgomery Terrace in Kilwinning.
Chief Superintendent Ruaraidh Nicolson, of Strathclyde Police, said: "Naturally, given the significant police presence here, people are bound to be concerned. I would like to reassure everyone who lives here that they are not at risk and that we are fully in control of the situation.
"I would expect that there will continue to be a fair amount of activity in this area throughout the day. I appreciate that this may be inconvenient to people who want to go about their daily business.
"I can assure you that we will be doing everything we can to keep disruption to a minimum and to get the local area back to normal as quickly as possible.
"I would say, though, that if anyone has any concerns, please speak to one of the officers at the scene or call your local police station."
Last month Strathclyde Police said the two packages sent to Lennon, and the two others, were "designed to cause real harm to the person who opened them".
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.
The Scotland-based group organises republican marches, including the James Connolly march through Edinburgh and the Bloody Sunday march every January in Glasgow. It also campaigns against racism and sectarianism in Scotland.
Early investigations suggested the fifth package was similar to those already being investigated, and was designed to cause "fear, alarm and real harm", police said.
All of the parcel bomb packages were found during March and last month.