Twenty-nine children who were taking their school exams in the Central African Republic have been killed in a crush after a nearby explosion caused panic, a hospital director told the BBC.

The blast, on the second day of the high-school finals on Wednesday, occurred at an electricity transformer, said Abel Assaye from the Bangui community hospital.

"The noise of the explosion, combined with smoke" caused alarm among the almost 6,000 students sitting the baccalaureate at a school in the capital, Bangui, local radio station Ndeke Luka reported.

President Faustin-Archange Touadéra has declared a period of national mourning.

He also ordered that the more than 280 who were wounded in the crush get free treatment in hospital.

Students from five different schools in the capital had gone to the Lycée Barthélémy Boganda to sit the baccalaureate exam.

The education ministry said the explosion happened after power was restored at the electricity transformer, located on the ground floor of the main building, that had been undergoing repairs.

"I also offer our sincere condolences to the parents of the affected candidates and wish a speedy recovery to the injured candidates," Education Minister Aurelien-Simplice Kongbelet-Zimgas said in a statement.

He also announced the suspension of further exams.

A female survivor spoke to the BBC.

"I don't even remember what happened. We were in the exam room and when I heard a noise, I immediately fell into a daze," she said. "Since then, I have had a pain in my pelvis that is causing me a lot of problems."

Radio France Internationale spoke to another student whose face was covered in blood after he had climbed out of a window.

Magloire explained that the blast happened during the history and geography exam.

"The students wanted to save their lives, and as they fled, they saw death because there were so many people and the door was really small. Not everyone could get out," he told RFI.

The CAR continues to face political instability and security challenges.

Government forces, backed by Russian mercenaries, are battling armed groups threatening to overthrow Touadéra's administration.