Preliminary results announced on Monday showed that Ashraf Ghani, a former World Bank official, won the second round of voting on 14 June. But his rival, Abdullah Abdullah, rejected the outcome, and claimed the vote was marred by widespread fraud.
Mr Abdullah also said US Secretary of State John Kerry would visit Kabul this Friday.
Thousands of Mr Abdullah’s supporters gathered in the Afghan capital yesterday, demanding he form a parallel cabinet and unilaterally assert his own rule – a move that would further fracture the fragile country.
Mr Kerry said there was no justification for violence or “extra-constitutional measures”.
“I have noted reports of protests in Afghanistan and of suggestions of a ‘parallel government’ with the gravest concern,” he said in a statement. “Any action to take power by extra-legal means will cost Afghanistan the financial and security support of the United States and the international community.”
Afghanistan is heavily reliant on foreign donors to fund everything from building roads and paying school teachers to security. The US pays the lion’s share of this international aid.
Observers fear a stand-off between Mr Abdullah and Mr Ghani could leave Afghanistan with no clear leader and plunge the country into disorder.
Mr Abdullah has accused president Hamid Karzai, who is stepping down after 12 years, of helping rig the vote in favour of Mr Ghani, describing it as a “coup” against the people.
The stand-off quashed hopes for a smooth transition of power in Afghanistan, a concern for the West as most US-led forces withdraw from the country this year.
There are concerns about how much Mr Abdullah would be able to control his supporters if the crisis escalated.
Thousands of his followers vented their anger in Kabul yesterday, chanting “Death to Karzai”, tearing down a large portrait of the outgoing leader and replacing it with an image of Mr Abdullah.
At the rally, Mr Abdullah, flustered by the size of the gathering, faced a roar of slogans demanding he immediately announce his own cabinet, but told supporters to be patient.
“We are the winner of this round of elections without any doubt,” he said.
“The people of Afghanistan call on me to announce my government. This was and is a demand from the people of Afghanistan … we cannot ignore this call. Once again I ask you to give me a few days to consult.”
The apparent softening of his tone comes after he spoke to Mr Kerry and US president Barack Obama.
“John Kerry will come to Afghanistan on Friday, and their promise was that they will be next to the people of Afghanistan in defending justice, fighting against fraud and revealing fraud,” Mr Abdullah said.
The Independent Election Commission said on Monday that Mr Ghani won the second round with 56.44 per cent of the vote, according to preliminary results. The tally might change when the final official numbers come out on 22 July.