The SNP leader made the announcement on Wednesday after more than eight years in the job.

She plans to remain in office until her successor is elected.

The SNP's national executive committee will meet on Thursday evening to draw up a timetable for a leadership race.

With no obvious successor, the party's first leadership contest in nearly 20 years could see a debate on future direction and strategy.

Possible replacements include Deputy First Minister John Swinney, Kate Forbes - who was finance secretary before her maternity leave - and Constitution Secretary Angus Robertson.

Health Secretary Humza Yousaf and Justice Secretary Keith Brown have also been suggested as potential candidates, but no one has yet signalled their intention to stand.

Ms Sturgeon made her announcement at a hastily convened news conference at her official Edinburgh residence, Bute House, but insisted it was a decision she had been weighing up for some time.

She said that in order to serve well, a politician needed to accept when it was time to make way for someone else.

"In my head and in my heart I know that time is now. That it's right for me, for my party and my country," she said.

Ms Sturgeon said her departure was not in response to the "latest period of pressure", which has included controversies over gender recognition reforms, trans prisoners and the strategy on independence.

She emphasised the huge pressures and sacrifices that came with serving in high office, adding: "I am a human being as well as a politician."

She intends to remain an MSP until at least the next Holyrood election.

The party's ruling body will now also have to decide on whether to go ahead with a special conference due to take place in March to discuss Ms Sturgeon's strategy of using the next general election as a de facto independence referendum.

The SNP's Westminster leader Stephen Flynn, who ruled himself out of the leadership contest, has called for the conference to be paused until a new leader is elected.

In her resignation speech, Ms Sturgeon said her party had an "array of talent" who could replace her as first minister.

The SNP's constitution says a candidate for party leader needs to have the backing of 100 members from at least 20 different SNP branches, with nominations already open.

If there is more than one candidate, a vote of party members will choose the new leader.

Michael Russell, the party's president, said he expected the process to be "shortened" and that it would be a "contested election".

He told Radio 4's PM: "I think that will be good for the SNP, to have different points of view contesting in a respectful way.

"I think we will decide that pretty soon and have a clear timetable that will take us forward."

Ms Sturgeon rose to power unopposed after the independence referendum in 2014, taking over from Alex Salmond who decided to resign following the vote to remain part of the UK.

She is the longest-serving first minister and the first woman to hold the position. She has worked as an MSP since the Scottish parliament was opened in 1999.

Originally from Irvine in North Ayrshire, she has campaigned for the SNP since she was a teenager.

In her resignation announcement she said she intended to remain active in politics, championing causes including Scottish independence and improving the life chances of children who have grown up in care.

Previously she has suggested she might consider becoming a foster parent. There has also been speculation that she might continue to play a role on the world stage with an organisation such as the United Nations.