Two people were killed Sunday as heavy weapons fire and clashes in the Muslim district of Bangui, the Central African Republic capital, marred a key constitutional referendum aimed at ending years of sectarian strife.

An AFP journalist saw the bodies of two people lying in a mosque in Bangui's PK5 Muslim district, while a hospital source said 20 others were wounded there, in clashes pitting supporters against opponents of the referendum seen as key to a return of normalcy in the divided nation.

Fire from heavy machine-guns and rocket launchers raged around a PK5 school where voters were waiting to cast their votes, prompting UN peacekeepers to move in to protect residents.

The proposed constitution would limit presidential tenure to two terms, fight institutional corruption and crimp the power of armed militias, blamed for years of chaos and terror.

If adopted, it would usher in the sixth republic since independence from France in 1960 and mark its 13th political regime -- underlining the chronic instability undermining the country.

The vote was seen as a test run for presidential and parliamentary elections due to take place December 27 to end more than two years of conflict between Muslim and Christian militias.

Some factions of the mainly Muslim Seleka force had threatened to block the vote, as had some "anti-balaka" Christian and animist militia supporters.

Among the latter were backers of ousted president Francois Bozize, whose candidacy for the presidential election was rejected by the constitutional court.

 'Dying to vote'

The Central African Republic plunged into its worst crisis since independence after longtime Christian leader Bozize was ousted by rebels from the Seleka force in March 2013, triggering a wave of violence with "anti-balaka" militias.

Despite the presence of 11,000 UN and French peacekeepers, part of the impoverished country remains out of bounds, under the control of either rebel chieftains or bandits.

Some Muslims however insisted on voting, with several dozen staging a protest Sunday morning at the entrance to MINUSCA's headquarters, holding posters that read: "We want to vote!"

"I want to vote, if I must die to vote that's OK," said Abakar, a voter waiting to cast his ballot at the Baya Dombia school in the PK5 district.

General Balla Keita, chief of the UN peacekeeping force MINUSCA, vowed to protect voters.

"We are here, we will stay with them on the battlefield. They (PK5 residents) will be able to vote, they will vote until nightfall if need be," Keita pledged.

Voting in other parts of Bangui got under way, albeit a couple of hours late, because staff and voting material were late to arrive.

In other parts of the country, several incidents were also reported, said an unnamed source in MINUSCA, sent in to quell fighting that has forced 10 percent of the population to flee the country.

Boycotts and threats

Very few voters turned out in Ndele, Birao and Kaga Bandoro, strongholds of Noureddine Adam's faction of the Seleka force, who called for a boycott and whose supporters fired shots and threatened residents.

In the western bastion of longtime leader Bozize, a Christian, shots were fired to dissuade people from voting and polling material looted.

The ballot comes two weeks after Pope Francis appealed to Muslims and Christians to live as "brothers and sisters".

Almost two million Central Africans had registered to vote in a population of 4.8 million, a clear sign of the widespread desire to return to a life of peace and normalcy.

Many of the 5,600 polling stations are located in remote areas accessible only by dirt roads. And of the 460,000 people displaced by the unrest living in camps across Central African Republic's borders -- many of them Muslims -- only 26 percent have been able to register.

The international community, which has been pouring aid into the country for over two years, was keen for the referendum as well as the follow-up elections to take place.

"These are make-or-break elections," said the International Crisis Group's Thierry Vircoulon.