Guinea's much-delayed presidential run-off will not be held as planned on Sunday, Guinea's election chief says.
Gen Toumany Sangare, who took over at the electoral commission on Wednesday, did not announce another date.
Rival supporters of the two candidates clashed on the streets of the capital, Conakry, after the delay was announced.
The UN warned that the long delay could raise the risk of instability, while the US said the poll was very important to Guinea's future.
The election is intended to be the first democratic poll in the West African country, after 52 years of authoritarian rule.
The military seized power in 2008, leading to two years of political upheaval.
The run-off vote was initially due to be held in July; this is the third time it has been postponed.
Supporters of the two candidates, Cellou Dalein Diallo and Alpha Conde, fought running battles in Conakry, and clashes were also reported in Kankan and Siguiri.
Witnesses quoted by Reuters news agency spoke of hearing gunfire as the security forces tackled protesters, but the situation was reported peaceful later in the day.
Gen Sangare, a Mali national, was appointed by the military leader after months of infighting and accusations of fraud at the electoral commission.
During Thursday's meetings at the commission, Gen Sangare discovered that neither the alpha-numeric voter cards nor the sealed voter envelopes had been distributed.
It was also revealed that computers meant for electronic vote-counting were stolen from the commission's premises earlier this week.
Meanwhile, agents and other temporary electoral staff have threatened to boycott the vote if salary arrears going back 11 months are not paid.
Gen Sangare also met interim Prime Minister Jean-Marie Dore, who urged the new electoral chief to tread carefully as he feared further violence.
On Friday, the UN and US both urged Guinea to set a new date as soon as possible.
The head of the United Nations Office for West Africa, Said Djinnit, told Reuters that a long delay could raise the risk of instability.
US State Department spokesman PJ Crowley said the poll was very important to Guinea's future, adding that Washington was "hopeful that the people of Guinea will avoid significant violence".
The country has been tense since the first-round vote in June and the run-off has been delayed - once in July because of investigations into electoral fraud and again in September because of clashes between rival supporters.
Earlier this week, two supporters of leading candidate Cellou Dalein Diallo were killed as police opened fire on crowds who had been demanding the removal of the former election commission head.
Both presidential candidates have given their backing to Gen Sangare.
Meanwhile, diplomatic sources confirmed to the BBC that troops from neighbouring Sierra Leone have been deployed to patrol the area near their common border.
A former prime minister, Mr Diallo is seen as the favourite to win the run-off. He took 44% of the votes in the first round - and claims he was denied overall victory only by fraud.
Mr Conde, a veteran opposition leader, won 18% of the vote - although he claims he was cheated out of some 600,000 ballots.
Correspondents say the fierce tensions between the two candidates' supporters has its origins in rivalry between Guinea's two largest ethnic communities. Mr Diallo is a Peul, while Mr Conde is a Malinke.
Despite their economic dominance, a member of the Peul community has never been president. The Malinke are heavily represented in the ruling military junta.
The first round was seen as Guinea's first democratic vote since independence in 1958, raising hopes of an end to military and authoritarian rule in the mineral-rich country.
Guinea is the world's largest exporter of the aluminium ore bauxite. It also has important deposits of iron ore, but it remains one of the poorest countries in West Africa.