Conditions in Liberian prisons violate basic human rights with inmates deprived of running water, food, beds and medicine, Amnesty International said in a report.

Appalling conditions were witnessed in four jails it visited in July, the rights group said in a statement, urging the government to take swift action.

"Inmates can suffer permanent damage to their physical and mental health as a result of their incarceration and most haven't even been convicted of a crime -- they're simply waiting for a trial date," Amnesty's Tawanda Hondora said on Wednesday.

None of the prisons visited had running water, "and the smell of sewage is overwhelming in most of the cells", added the statement.

"Severe overcrowding in some prisons has a serious effect on the health and safety of prisoners."

Amnesty estimates there are some 1,500 to 1,700 prisoners in Liberia's 15 jails, including 50 women and several children who had been awaiting judgment for several months.

Monrovia Central Prison, Liberia's largest, was designed to hold 374 inmates, but in July had 839, the rights group said.

In one block, Amnesty had found two-by-three metre (6.5-by-10 feet) cells housing eight men with only a small window for ventilation. Inmates had to sleep in shifts as there was not enough space for them to all lie down at the same time.

In some cells, inmates strung hammocks they had made of grain sacks three or four metres above the ground, sometimes falling out at night and breaking ribs or dislocating shoulders.

Healthcare was also lacking, said the statement, with malaria, skin infections and eye problems often untreated.

While some progress had been made, "... the government has a clear and binding obligation not to expose prison inmates to conditions that constitute cruel, inhuman and degrading treatment", Hondora said.

"Immediate action is needed to improve conditions, including access to health care, for Liberia's prison inmates."

Liberia will hold presidential and legislative elections next month, only the second since the country emerged from its 1989-2003 civil war which virtually destroyed the its economy.