Doctors from the scene of violent anti-government protests in Yemen's capital said that what was originally thought to be tear gas fired by government forces on demonstrators might instead have been a form of nerve gas, which is forbidden under international law.

Military personnel opened fire on Tuesday night and used what was originally assumed to be tear gas to disperse a group of demonstrators who were trying to bring additional tents into the protest area outside Sanaa University.

According to witnesses, the soldiers fired warning shots into the air before shooting gas - and in some cases live bullets - into the crowd, killing one and injuring at least 50.

Earlier reports indicated that the gas used was tear gas, but doctors who have been treating the wounded refuted that claim today.

"The material in this gas makes people convulse for hours. It paralyses them. They couldn't move at all. We tried to give them oxygen but it didn't work," said Amaar Nujaim, a field doctor who works for Islamic Relief.

"We are seeing symptoms in the patient's nerves, not in their respiratory systems. I'm 90 per cent sure its nerve gas and not tear gas that was used," said Sami Zaid, a doctor at the Science and Technology Hospital in Sanaa.

Mohammad Al-Sheikh, a pathologist at the same hospital, said that some of the victims had lost their muscular control and were forced to wear diapers.

"We have never seen tear gas cause these symptoms. We fear it may be a dangerous gas that is internationally forbidden," Al-Sheikh said.