Mr Leonard, a former GMB union organiser who was elected to Holyrood in 2016, is competing against fellow MSP Anas Sarwar to become Kezia Dugdale’s successor, after she dramatically quit the post after two years in charge.
While Mr Sarwar, a former MP and previous deputy Scottish Labour leader, is seen as being in the moderate wing of the party, Mr Leonard is more to the left and closer politically to UK leader Jeremy Corbyn.
Mr Leonard, a Central Scotland MSP, will formally launch his leadership bid at an event in Glasgow on Saturday.
He is expected to say: “We need wholesale and radical change and we need a completely new approach to tackle what appear to be the intractable problems in our housing system, of an inequality that pervades all aspects of Scottish life.
“The truth is that only the Labour Party can, and will, solve these problems and transform the lives of the people of Scotland.
“Only Labour, with radical leadership in Scotland and Jeremy Corbyn’s principled leadership across the UK, will deliver the real change needed to tackle Scotland’s deep rooted social and economic problems.
“Twenty years since the devolution referendum we have every right to be angry. No wonder people are discontented. They are hungry for change.
“But the change they crave will find no answer in nationalism or patriotism, Scottish or British, and every answer in socialism and democracy.”
Mr Leonard’s campaign launch comes less than 24 hours after Mr Sarwar formally began his drive to become the next leader of the party.
He told supporters in Glasgow that the contest is about electing the next First Minister of Scotland as he spoke of his wish to build a more ‘’open, outward-looking, diverse nation’’.
Mr Sarwar told Labour supporters they could choose ‘’to focus on the divisions of the past, or to build a fairer future for all’’.
Addressing a packed gathering in First Minister Nicola Sturgeon’s constituency, he said: ‘’We have a choice to protest about the ills in our country or to defeat inequality in power.
‘’We have a choice to continue to divide, or to bring our party and our country together.
‘’I don’t want to be a Scottish Labour leader who just talks about fighting inequality, creating opportunity, ending austerity and building a fairer society.
‘’I want to deliver that equality, opportunity and fairness as the next First Minister of Scotland.’’
The result of the contest will be announced in November.