Amnesty International on Tuesday slammed an Oman court's conviction of a group of men and a woman on charges of defaming the country's ruler Sultan and for cyber crimes as an attack on free speech.

"These sentences are the latest phase in the Omani government's orchestrated crackdown on freedom of expression and assembly," said Philip Luther, Middle East and North Africa Programme Director at Amnesty.

"All charges levelled against activists merely for engaging in peaceful activism must be dropped," he said in a statement.

"If anyone is imprisoned on the basis of such charges, we would regard them as prisoners of conscience and call for their unconditional and immediate release," he said.

A court on Monday sentenced five men and a woman to one year in prison each for defaming Oman's ruler Sultan Qaboos and for cyber crimes, their lawyer said.

The six were part of a group of 36 intellectuals and bloggers demanding political reforms who were arrested at the start of June and subsequently released.

Four activists were sentenced on July 9 to jail terms of between six months and a year for defaming Sultan Qaboos, but were freed on bail pending appeal.

The latest six people to be convicted were also released on bail pending their appeal hearing on September 15, lawyer Yaqub al-Kharusi said.

Paris-based watchdog Reporters without Borders and New York-based Human Rights Watch have also denounced the arrests.

Oman was hit by a wave of protests last year demanding political reforms and riot police dispersed the demonstrations with force.