BANGUI, Central African Republic (AP) — Gunfire, a grenade explosion and threats of violence in Central African Republic on Sunday prevented many from voting in a constitutional referendum seen as a test of whether national elections can take place later this month.

Sunday's vote comes ahead of presidential and legislative elections on Dec. 27 that already have been delayed several times. Concerns about security and voter turnout remain high, even though a visit by Pope Francis two weeks ago brought a momentary lull in violence between Christian and Muslim militias.

An attack on a voting station in the PK5 neighborhood by gunmen killed at least one person and injured six others, said a Muslim community leader Ousmane Abakar. Gunshots had been heard since early Sunday in the Bangui neighborhood, he said.

A grenade exploded near a voting center in the northern Gobongo neighborhood of Bangui, injuring three people, residents said. In Kaga Bandoro in the country's north, an armed group threatened to kill people who went to vote. In the central town of Bria, voting materials were set on fire, said Col. Mahamat Deya.

The referendum for which 2 million people were registered to vote at more than 5,500 stations continued elsewhere, though there were reports of missing voter cards.

If approved by voters, the new constitution will create a Senate and safeguard for freedom of worship and religious freedoms.

United Nations Secretary-General Ban Ki-moon called for a peaceful and credible vote. "The referendum is a significant milestone towards the end of the transition in the Central African Republic which will lay new foundations for a stable future for the country and its people," he said in a statement.

Central African Republic descended into conflict in 2013 when Muslim Seleka rebels overthrew the Christian president. That ushered in a brutal reign in which Muslims committed atrocities. When the rebel leader left power the following year, a swift and horrific backlash by the Christian anti-Balaka militia against Muslim civilians followed and sectarian violence has continued.

The U.N. mission in Central African Republic currently has more than 11,000 troops and police are trying to calm the deadly violence.