Brazil has called for more UN troops to be sent to Haiti to curb a wave of unrest in which more than 40 people have been killed in a fortnight.
Brazilian peacekeepers feel outnumbered in Haiti
Foreign Minister Celso Amorim said the situation was worsening because the UN force, which Brazil leads, was operating at less than half-strength.
Haiti's interim government blames supporters of exiled president Jean-Bertrand Aristide for the violence.
Mr Aristide's loyalists are planning protests in the capital on Friday.
The protests in Port-au-Prince look set to coincide with the arrival of some of the former soldiers who helped oust the president in February.
The soldiers say they want to help restore order, but the UN mission says it will not work with them.
Correspondents say there are fears the presence of the former soldiers could spark a confrontation with Mr Aristide's supporters.
A rally in the city on 30 September was marred by riots and the killing of two protesters by police.
Correspondents say the government of interim Prime Minister Gerard Latortue does not have the resources to impose order.
The UN force is faring little better. It has just 2,600 soldiers - a fraction of the 6,700 troops and 1,600 police authorised for Haiti by the UN Security Council, Mr Amorim says.
In recent weeks the peacekeeping mission has been further stretched by the need to keep order in the hurricane-hit northern city of Gonaives.
US staff to leave
Mr Amorim has made an urgent appeal for more troops.
"As long as we don't have the increase in forces, it's going to be very difficult," he said on Thursday.
The violence in Port-au-Prince has claimed dozens of lives
He added that the force should reach it full planned size.
In another sign that law and order is breaking down, the US said it would allow its non-essential diplomatic staff to leave Haiti.
On Wednesday one of the commanders who led the uprising said former soldiers were converging on Port-au-Prince to restore order.
The exiled president insists he is still Haiti's legitimate leader and accuses the US of having helped to oust him.