BANGUI (Reuters) - Voters in Central African Republic braved heavy fighting and intimidation by armed groups to cast their ballots on Sunday in a constitutional referendum seen as a crucial step toward ending nearly three years of violence.

The former French colony descended into chaos in early 2013 when mainly Muslim Seleka rebels seized control in the majority-Christian nation, committing abuses that led to reprisals by Christian anti-balaka militias.

Thousands were killed in the ensuing inter-religious violence and roughly one-in-five Central Africans have been displaced in a de facto partition.

Clashes broke out on Sunday after U.N. peacekeepers from the MINUSCA mission, brought in to protect poll workers and voters, came under fire in PK5, an enclave of Muslims who have refused to flee the capital Bangui despite attacks by Christian militias.

Fighters armed with automatic weapons and rocket-propelled grenades launched repeated attacks in what MINUSCA’s force commander General Bala Keita said was an attempt by “spoilers” to block the vote.

“Sometimes we stop the whole exercise to fight back and then we resume the voting. That’s happened three or four times,” he said. “It’s a war zone. I think it’s something extraordinary. We’re helping people to vote while we’re fighting the others.”

Witnesses said two people were killed in PK5. Medical charity Medecins Sans Frontieres said it admitted 23 people with gunshot and grenade wounds during the day in the capital.

A U.N. official said two peacekeepers were wounded in the fighting in Bangui and another was injured in the central town of Bria.

The expected approval of the draft constitution would pave the way for Dec. 27 polls to elect a new president and parliament, restoring democratic rule in the wake of two successive transitional governments.

The U.N.’s aggressive intervention in PK5 came after residents of the enclave, visited by Pope Francis during a trip to Bangui last month, marched to MINUSCA’s headquarters to complain they were unable to cast their ballots.

“If we have these, it’s so we can vote,” said resident Karim Bashir Abakar, holding up his voter card. “They’re trying to stop us from voting. We’ve had enough of guns. We want to go vote.”

“SICK AND TIRED OF CONFLICT”

Despite the deployment of the 11,000-strong MINUSCA last year and the presence of French soldiers, sporadic violence continues.

In a statement on Sunday, one of the main factions of the Seleka coalition said conditions, including the return of refugees, were not in place to allow polls to go ahead.

“CAR (Central African Republic) isn’t ready to organize inclusive, democratic, credible, safe and transparent elections,” it said, calling for a new transitional government and the cancellation of both the referendum and the elections.

There were attempts to derail Sunday’s vote in several parts of Bangui as well as in Kaga-Bandoro, a stronghold of Seleka commander Noureddine Adam, where local radio said fighters ransacked polling stations.

In Bossangoa, a hotbed of support for deposed president Francois Bozize, gunfire from the early morning forced voters to stay home, according to Radio Ndéké Luka.

There were also problems in the northeastern town of Birao, said Diane Corner, the head of MINUSCA. She estimated that voting had nonetheless taken place normally in 80 percent of polling stations nationwide amid high turnout among the nearly 2 million registered voters.

“There were some pretty dramatic events, but in a small number of places,” she said. “The vast majority of Central Africans are sick and tired of the conflict. They’re sick and tired of the people who want to perpetrate conflict.”

Polling stations closed and poll workers immediately began counting ballots, but elections commission head Marie-Madeleine N’Kouet said voting would likely be extended into Monday in areas where logistical problems had hobbled the process.

The proposed constitution limits the power of the president and increases that of the parliament, creating a senate to compliment the already existing national assembly. It also establishes a Special Criminal Court to try serious crimes.