A Myanmar court has found democracy leader Aung San Suu Kyi guilty of violating her house arrest by allowing an uninvited American to stay at her home. The head of the military-ruled country ordered her to serve an 18-month sentence under house arrest. The 64-year-old Nobel Peace Prize laureate has already been in detention for 14 of the last 20 years, mostly under house arrest, and the extension will remove her from the political scene while the country holds junta-organised elections next year. The sentence drew immediate criticism from around the world, with the British prime minister Gordon Brown calling it "monstrous". Suu Kyi looked alert but tired during the 90-minute court session. She stood as the verdict was announced then thanked foreign diplomats for attending her trial. "I hope we can all work for peace and prosperity of the country," Suu Kyi said in a soft voice to diplomats seated nearby. She was then led out of the courtroom. Officials said she was driven back to her lakeside villa to serve the house arrest. They spoke on condition of anonymity because of the sensitivity of the high-profile case. The American, John Yettaw, was sentenced to seven years in prison, including four with hard labour. Suu Kyi had faced up to five years in prison for allowing Yettaw to stay for two nights after he swam across a lake to reach her. The court initially sentenced Suu Kyi to a three-year prison term with hard labour. But after a five-minute recess, the home minister Maj Gen Maung Oo entered the courtroom and read aloud a special order from junta chief Senior Gen Than Shwe. The order said Senior Gen Than Shwe was cutting the sentence in half to 18 months and it could be served under house arrest. Senior Gen Than Shwe's signed the order on Monday. He also reduced the sentences of Suu Kyi's two female house companions, Khin Khin Win and Win Ma Ma, to 18 months. Both are members of her political party. Senior Gen Than Shwe said he reduced the sentences to "maintain peace and tranquility" and because Suu Kyi was the daughter of Aung San, a revered hero who won Myanmar's independence from Britain. Suu Kyi's trial has sparked international outrage and calls for her release and that of Myanmar's more than 2,000 other political prisoners. The sentence sparked angry reaction from across the globe. "The facade of her prosecution is made more monstrous because its real objective is to sever her bond with the people for whom she is a beacon of hope and resistance," Mr Brown said, calling the verdict a "purely political sentence" aimed at keeping her out of the 2010 elections. Burma Campaign UK, an activist group, called for a global arms embargo against Myanmar and said the junta was "determined to silence all pro-democracy voices in the country in the run up to rigged elections". Suu Kyi's international lawyer, New York-based Jared Genser, said her most recent period of detention violated Myanmar's own laws. "The real question is how the international community will react - will it do more than simply condemn this latest injustice?" he said. The 53-year-old Yettaw, of Falcon, Missouri, was returned to Insein prison, the site of the trial, on Monday night after hospitalisation for epileptic seizures. The court sentenced him to three years in prison for breaching Suu Kyi's house arrest. Yettaw was also sentenced to three years in prison with hard labour for an immigration violation and to another one-year term with hard labour for swimming in a restricted zone. It was not immediately clear if the prison terms would be served concurrently. Yettaw, a devout Christian, earlier told his lawyer that he swam to Suu Kyi's residence to warn her of an assassination attempt that he had seen in a vision. Yettaw was hospitalised last Monday after suffering seizures. He reportedly suffers from epilepsy, diabetes and other health problems, including post traumatic stress disorder from his service in the US military. The home minister Maung Oo said that under house arrest rules for Suu Kyi and her two companions, they would be allowed medical care by an authorised doctor, access to government media and visitors who received prior authority. *AP