The coalition backing former rebel leader Jean-Pierre Bemba has rejected results from the Democratic Republic of Congo's presidential run-off.
Joseph Kabila (l) is leading Jean-Pierre Bemba (r)
President Joseph Kabila is poised for victory with 90% of the votes counted, a preliminary result count shows.
Mr Kabila has 60%, while Mr Bemba, who has been vice-president since a peace deal, has 40%, according to the Independent Electoral Commission (CEI).
The Bemba camp said victory "was being stolen from the Congolese people".
It said in a statement, that if the CEI had cheated, they would not feel bound to comply with an earlier promise to respect the outcome of the election.
"The Union for the Nation will not accept an electoral hold-up that aims to steal victory from the Congolese people," the coalition said in a statement
Votes counted: 90%
The vote is the first following DR Congo's five-year conflict, but tension is high in the capital, Kinshasa, following weekend clashes in which four people were killed.
The BBC's Mark Doyle says the challenge to the results could be highly dangerous, especially in the capital, Kinshasa, which is a Bemba stronghold and just a few thousand United Nations peacekeepers are deployed.
The UN is backed by a special European military mission but the numbers of European troops may not be enough to contain any widespread unrest if that occurs, he says.
The CEI is not declaring a winner until it investigates allegations of fraud by Mr Bemba and his supporters.
CEI spokesman Dieudonne Mirimo said Mr Bemba's party had now submitted five official complaints.
Following Saturday's violence, the police arrested 337 homeless people, including 87 children, the government says, blaming them for starting the trouble.
Eye-witnesses say that security forces loyal to the two candidates exchanged gun- and mortar-fire.
United Nations observers say the election is the most significant in Africa since Nelson Mandela was elected as South Africa's president in 1994.
At least four people were killed in Saturday's clashes
They are seen as the country's first free elections since independence in 1960.
The commission has until 19 November to announce the results and stresses that no "trend projection" can be made on the basis of the provisional results.
The first round of elections showed a regional divide, with Mr Kabila gaining a landslide in the Swahili-speaking east, while Mr Bemba got most support in the west, where Lingala is the common language.
The world's largest peacekeeping force - 17,000-strong - is in DR Congo, tasked with ensuring security.
At least 23 people were killed in gun battles between security forces loyal to the two men in the capital, Kinshasa, after the announcement of first round results.
Mr Kabila won 45% of the vote, while Mr Bemba got 20%.
International observers generally praised the vote as being well-run, despite some disruptions in the north-east of the country.
The election was intended to close the door on decades of dictatorship and conflict.
Counting the votes is a time-consuming process as all the ballot papers had to be transported from sometimes remote locations to compilation centres.
DR Congo is two-thirds the size of western Europe and has just 300 miles of paved roads.
The country's rich reserves of minerals such as gold, diamonds and coltan - used in mobile phones - have attracted a series of armed groups, both Congolese and foreign, intent on looting.