Their charges include riot, rioting injuring buildings and willful damage. Additional charges of common assault were levelled against four men, Nauru government spokesman Rod Henshaw said.

All are being tried under Nauruan law, and the charges carry possible jail terms of up to seven years.

Two of the group have voluntarily returned to Iran after the incident.

The group's last appearance in court on November 18 was an emotional one, with asylum seekers at first refusing to participate in proceedings over concerns they would not get a fair trial.

Then, the men had refused to leave their bus outside Nauru's tiny courthouse for two hours after being told they would be represented by a man they had not met, the island's public defender.

Local lawyer Pres-Nimes Ekwona agreed to represent the men at that hearing, and it is understood he represented the men again on Monday, although he warned last month that he would be unable to represent all the defendants.

Four men were unable to appear in court for medical reasons, Mr Henshaw said.

The asylum seekers were not required to enter a plea, but it is expected they will be required to plead guilty or not guilty when they reappear at court on January 14.

A spokesman for the Salvation Army said Salvation Army workers and volunteers were in court to support the men. The agency would work to help the men get legal representation, he said.

Nauruan photographer Clint Deidenang reported on Twitter that there had been a heavy security presence at the hearing.