John Swinney has launched his campaign to succeed Humza Yousaf as SNP leader and Scotland's first minister.
The highly experienced former cabinet secretary, who previously led the party between 2000 and 2004, has been backed by several senior SNP colleagues.
He told a press conference in Edinburgh he aimed to "unite Scotland for independence".
Former finance secretary Kate Forbes will announce her decision on a leadership challenge later on Thursday. She did not attend Mr Swinney's launch event in Edinburgh and was said to be “still weighing up her options”.
Mr Swinney, 60, announced his decision at a speech in the city's Grassmarket. His campaign posers displayed the slogan "uniting for independence".
He told supporters: “I want to build on the work of the SNP government to create a modern, diverse, dynamic Scotland that will ensure opportunity for all of her citizens.
“I want to unite the SNP and unite Scotland for independence.”
The former party leader, who served as Nicola Sturgeon's deputy for almost nine years, announced late on Wednesday that he was preparing to make a statement as speculation increased about a run for the leadership.
He has been backed by SNP Westminster leader Stephen Flynn, deputy leader Keith Brown and cabinet ministers Neil Gray, Jenny Gilruth, Shirley-Anne Somerville and Mairi McAllan.
Ms McAllan introduced Mr Swinney at the launch event, describing him as a "steadfast" defender of Scotland's interests.
Cabinet ministers Angus Robertson and Fiona Hyslop also attended the campaign launch.
Ms Forbes is the only other SNP figure to have said she is considering a leadership challenge.
She pulled out of a meeting of the Scottish Parliament committee on Thursday morning.
If two candidates receive enough nominations from party branches, a ballot of party members will be held.
If there is only one candidate, they will automatically win the leadership.
Party officials have confirmed nominations will close on Monday, with any contest ending on 27 May.
