The President of the Democratic Republic of Congo, Joseph Kabila, has given the forces of his presidential rival 48 hours to leave Kinshasa.
Soldiers have been stationed across Kinshasa
Mr Kabila has been declared the provisional winner of recent polls.
However, his rival, Jean-Pierre Bemba, has complained of fraud. The Supreme Court is to rule on the claims shortly.
Mr Bemba, a former rebel leader, has an armed guard of around 600 men, the UN says. Some of his men have reportedly started leaving their base in Kinshasa.
The BBC's Arnaud Zajtman in Kinshasa says tension is high in the capital.
Soldiers of the Congolese army are now deployed at strategic points and this has created fear of further fighting, he says.
Security forces loyal to the two men clashed in August, leaving at least 23 people dead in the capital, Kinshasa.
The capital is a stronghold of Mr Bemba.
The armoured vehicles of the UN peacekeeping force and European troops are also patrolling the city, but a UN spokesman said disarming soldiers by force was not part of its mandate.
On Tuesday, a protest by Mr Bemba's supporters outside the Supreme Court led to violence.
The court was set on fire and its hearings had to be suspended.
Mr Bemba has challenged the election result in court
The court must still confirm the provisional results, which gave Mr Kabila 58% of the vote.
A spokesman for Mr Bemba said he was unaware of any ultimatum for the guards to leave.
As a vice-president, Mr Bemba, is allowed to have 100 armed guards.
But one Western diplomat told the AFP news agency Mr Bemba had agreed to move some guards to Maluku, 80km (50 miles) east of Kinshasa.
The elections were supposed to draw a line under a five-year conflict in which some four million people died.
The polls were organised under the terms of a 2002 peace deal that drew in the armies of nine other African countries.
The world's biggest peacekeeping force - some 17,000 men - is in DR Congo to prevent unrest.