A stampede at a church gathering in Liberia's capital Monrovia killed 29 people overnight, the deputy information minister told state radio on Thursday.

The incident occurred during an all-night Christian worship event at New Kru Town, a neighbourhood on the outskirts of the capital, Jalawah Tonpo said.

"The doctors said 29 persons died and some are on the critical list," Tonpo said, calling into state radio from a nearby hospital. "This is a sad day for the country."

Exodus Morias, a resident who attended the event, told Reuters the stampede began after a group of armed men rushed the crowd in an attempt to stage a robbery.

"We saw a group of men with cutlasses and other weapons coming toward the crowd," Morias said. "While running, some people dropped and others fell on the ground and walked over them."

Bands of Liberian street gangs known as Zogos commonly commit robberies with machetes and other small weapons.

Police spokesman Moses Carter declined to comment on what caused the incident. He said an investigation is under way.

President George Weah, who is expected to visit the site on Thursday afternoon, declared a three-day period of national mourning and said the Liberian Red Cross and Disaster Management Agency had been called in to assist victims, his office said.