The EU vowed to hit Myanmar's junta with more sanctions yesterday and Britain demanded an arms embargo as Western governments and rights groups reacted with outrage at the sentencing of Aung San Suu Kyi.

After a Yangon court ordered the Nobel laureate to stay under house arrest for a further 18 months, protesters descended on Myanmar's diplomatic missions to denounce a trial described by Amnesty International as "political theatre".

With the sentence effectively ruling out any possibility of the 64-year-old standing in polls next year, there were immediate calls for a hardening of sanctions against the military rulers who prevented Ms Suu Kyi from taking power after party won elections in 1990.

"The EU will respond with additional targeted measures against those responsible for the verdict," the EU Swedish presidency said in a statement on behalf of the 27-nation bloc.

"In addition, the EU will further reinforce its restrictive measures targeting the regime of Burma/Myanmar, including its economic interests," it added.

Gordon Brown, Prime Minister of the former colonial power Britain, said he was "saddened and angry" at the verdict in the "sham trial".

Mr Brown said her "monstrous" prosecution, designed to stop her from taking part in next year's planned elections, meant the poll would have no legitimacy.

"This is a purely political sentence designed to prevent her from taking part in the regime's planned elections next year," Mr Brown said in a statement.

"So long as Aung San Suu Kyi and all those political opponents imprisoned in Burma remain in detention and are prevented from playing their full part in the political process, the planned elections in 2010 will have no credibility or legitimacy."

The British premier said it was time an arms embargo was slapped on the junta by the UN Security Council, whose permanent members include Myanmar's traditional allies in China.

"I also believe that the UN Security Council - whose will has been flouted - must also now respond resolutely and impose a worldwide ban on the sale of arms to the regime," said Mr Brown.

French President Nicolas Sarkozy said new sanctions had to hit the junta where it hurt.

"The President is asking the EU to respond quickly by adopting new sanctions against the Burmese regime," said a statement from his office.