The United Nations has urged the government of the Democratic Republic of Congo (DRC) to allow peaceful rallies on the eve of expected anti-government demonstrations called by opposition and civil society groups.

In a statement issued on Tuesday, the UN peacekeeping mission, MONUSCO, called on DRC authorities "to respect the fundamental freedoms enshrined in the Congolese Constitution, including freedom of assembly and of demonstration".

It also said that authorities should "instruct defence and security forces to respect the principles of necessity, proportionality and legality, consistent with international standards".

The MONUSCO call came amid fears of violence in a country that has seen widespread anger over what some consider as President Joseph Kabila's refusal to relinquish power after his second full term ended in December 2016.

A presidential election was meant to take place in November 2016, but officials said the vote was postponed because of deadly clashes in the Kasai region and logistical hurdles. Anti-Kabila protests have been met with a heavy police response since September 2016, and often turned violent.

On Sunday, the Independent National Electoral Commission (CENI) set a date for the new election: December 23, 2018.

Following the announcement, rights groups and opposition politicians raised fresh concerns as Kabila looks set to remain in office for another 14 months, having held power for the past 16 years.

Opposition leader Felix Tshisekedi called for protests, writing on Twitter that the election commission's calendar "will sound the death knell of the regime".

Fight for Change (LUCHA), an activist group, said CENI's calendar was "fanciful" and also called on people to protest peacefully against the "shameful manoeuvre to save more time in Kabila and its regime in order to fulfill their will to remain indefinitely in power".

In its statement, MONUSCO also called demonstrating citizens to "abstain from resorting to any form of violence",

"In the current context of political transition, it is important to allow all voices to express themselves calmly and peacefully," said Maman Sidikou, head of MONUSCO.

"In accordance with its mandate, MONUSCO reserves the right to monitor events and report on any human rights violations," he added.

Kabila had taken power after his father was assassinated in 2001, and was elected in 2006 and 2011.

According to the constitution, Kabila cannot seek a third term.