At least two people were killed and 20 others wounded when heavy fighting broke out in a Muslim enclave of Central African Republic's capital Bangui as a vote in a constitutional referendum was under way on Sunday.

Gunfire and the explosion of rocket-propelled grenades were heard in the PK5 neighbourhood, witnesses told the Reuters news agency.

The charity Medecins Sans Frontieres (MSF) said it treated 12 people for gunshot wounds.

 UN troops heading to the Central African Republic

Following months of relative calm in Bangui, clashes in late September sparked a fresh wave of inter-religious fighting that has killed more than 100 people in and around the PK5 neighbourhood, according to rights campaigner Human Rights Watch.

Residents of the isolated enclave - which welcomed Pope Francis during a trip to Bangui last month - marched to the headquarters of the country's UN peacekeeping mission, MINUSCA, to complain that they were unable to cast their ballots.

The mission responded by sending soldiers to protect voters and poll workers in PK5.

"Today was a truly remarkable day," a UN official, who spent the day at PK5, told Al Jazeera.

"The courage and determination of the Central African people was on full display. Despite determined attacks, despite taking casualties and fatalities inflicted by spoilers, those that had lined up to vote simply refused to leave until they had cast their ballot. It was inspiring and humbling."

The UN official said that three peacekeepers - two in Bangui and one in the central town of Bria - were also wounded on Sunday helping protect the vote.

Key vote

The new constitution - seen as crucial to restoring stability in the restive country - is set to replace a transitional charter that currently governs the former French colony.

About one-quarter of CAR's 4.7m population have been displaced since March 2013, when Muslim Seleka rebels overthrew Christian president Francois Bozize.

Fighting has been ongoing between armed Christian groups and Muslim fighters since then, despite the deployment of the 11,000-strong UN force last year.

If approved by voters, the new constitution will create a Senate and safeguard for freedom of worship and religious freedoms and will bar members of current interim government from standing in December 27 legislative and presidential elections.

It will also limit the presidency to two terms.

The landlocked nation is currently led by interim President Catherine Samba-Panza, a Christian, and Prime Minister Mahamat Kamoun, a Muslim.

Two million people were registered to vote at more than 5,500 stations.

Voting got off to a late start in many neighbourhoods in the sprawling, riverside capital, but queues formed in front of polling stations across the city.

"We received all the necessary materials for voting yesterday, and it was all monitored by MINUSCA agents," said Fabien Kodou, a member of the National Elections Authority.

"This morning we started work very early. We were missing certain things, including the electoral list. It's quite disorganised and there's nothing we could do about it. But we're trying our best and at least we had a good start."

Results are expected in the next three days.

Source: Al Jazeera and agencies