Cristina Fernandez de Kirchner has left the Argentine presidency, but is refusing to attend a ceremony to hand over to her successor Mauricio Macri.
The outgoing left-wing leader and the centre-right president-elect disagree over where the ceremony should be held.
Ms Fernandez bid farewell to a large crowd of her supporters in an emotional speech on Wednesday.
She urged people to take to the streets if they felt betrayed by the new centre-right government.
Mr Macri has now arrived at Congress following a horseback parade that passed thousands of people who were lining the streets. He will take his oath of office before legislators at noon local time (15:00 GMT).
Following his inaugural speech, he will then travel to the presidential palace where he will receive the presidential sash and baton.
Ms Fernandez had insisted that the handover should also take place in Congress, where her party holds a majority of seats.
She argued that both she and her husband and predecessor in office, Nestor Kirchner, had received the symbols of power in Congress and it had therefore become a tradition to be followed.
Mr Macri argued that according to presidential protocol, the handover should be held in the presidential palace, as it did before 2003.
Local media reported that Mr Macri's decision was probably driven not just by tradition but also by a concern that followers of Ms Fernandez could disrupt the ceremony in Congress.
Aides to Mr Macri said they feared her party could fill the gallery with her hardcore supporters.
Annoyed by Mr Macri's insistence, Ms Fernandez announced she would skip both ceremonies altogether.
Mr Macri's party in turn sought a court injunction affirming that Ms Fernandez's term ended at midnight on Wednesday to settle the matter.
As a result, power was temporarily transferred to Senate Speaker Federico Pinedo, who is acting as head of state for 12 hours until Mr Macri's inauguration.
Argentines have been mocking what many of them see as an unbecoming row over protocol.
Many posted unflattering photographs of Ms Fernandez on Twitter under the hashtag CFKverguenzaglobal (CFKglobalshame).
Ms Fernandez and Nestor Kirchner held power in Argentina for those 12 years.
'President for all Argentines'
In his inaugural speech, Mr Macri is expected to call for unity and reconciliation.
The conservative politician is not only inheriting a nation divided into supporters and opponents of Ms Fernandez but also burdened with a host of economic problems, correspondents say.
The new president's priorities
The current administration has been repeatedly accused of being less than transparent about key statistics such as the real rate of inflation.
The soon-to-be president has repeatedly said he could not yet expand on the detail of his economic plans until he was able to ascertain exactly how critical the situation was.
While his middle-class supporters will be expecting a more liberal economic climate, less well-off working class Argentines will be hoping the new administration protects welfare programmes introduced by the previous government.
Mr Macri indeed promised to be a "president for all Argentines", knowing the powerful labour unions will become restless if economic reform turns out to mean cuts.
Without saying so directly, Mr Macri made it abundantly that Ms Fernandez's combative style had damaged Argentina's image abroad.
Rebuilding relations with neighbours in Latin America and allies further afield, is another job at the top of his "to do" list.
Official figures suggest inflation is running at almost 15% but independent analysts put it much higher, at nearly 25%.
Argentina has suffered badly from a slump in commodity prices and foreign currency reserves have plummeted, making attracting external investment difficult.
The country defaulted on its debts last year for a second time in a dispute with hedge funds.
But Ms Fernandez has defended her record in an address to tens of thousands of cheering supporters outside the Casa Rosa presidential palace.
"We believe in what we have achieved so we need to have a positive attitude to ensure that these things will not be destroyed," she said.
"When you feel that those who you trusted and voted for have betrayed you, take up your flags," she added.
She is revered by some Argentines for expanding welfare benefits, nationalising some companies and introducing new civil rights such as gay marriage.
But critics say she created a culture of handouts and clogged Latin America's third-largest economy with interventionist policies.