A court in Myanmar sentenced the pro-democracy leader Daw Aung San Suu Kyi to 18 months of additional house arrest on Tuesday, ensuring that she would remain in detention, with limited communications, through a parliamentary election that is scheduled for next year.

The sentencing stoked the anger the world has shown over the continued detention of a woman who has become an international symbol of nonviolent resistance. President Obama said the sentence violated “universal principles of human rights,” and rights groups and foreign governments called Mrs. Aung San Suu Kyi’s conviction “reprehensible,” “brutal” and “monstrous,” and repeated their demand for her immediate release, as well as the release of an estimated 2,100 other political prisoners.

Playing up a moment of suspense, the court first sentenced Mrs. Aung San Suu Kyi, 64, to three years of hard labor for violating the terms of her house arrest, which she has lived under for 14 of the past 20 years. Moments later it reduced the sentence and sent her home from the prison where she had been held since the trial began three months ago.

The case stemmed from a strange and now notorious episode in which an American adventurer swam across a lake on May 3 and spent two days at her villa, claiming that he had come to save her from assassination. The American, John Yettaw, 53, was sentenced to seven years of prison and hard labor for breaching the rules of her house arrest and for violations of immigration law and local ordinances. Mr. Yettaw, who had suffered recently from what were described as epileptic seizures, was removed from the courtroom immediately after his sentence was read.

Two women who have lived with Mrs. Aung San Suu Kyi during her house arrest received the same three-year sentence and the same commutation and were expected to return with her to her lakeside villa.

Mr. Obama, in a statement, said that the conviction and sentencing “on charges related to an uninvited intrusion” was “an unjust decision” and that she and other political prisoners were “denied their liberty because of their pursuit of a government that respects the will, rights and aspirations of all Burmese citizens.”

“They, too, should be freed. Suppressing ideas never succeeds in making them go away,” his statement said.

Speaking in Goma, Congo, Secretary of State Hillary Rodham Clinton said: “She should not have been tried. She should not have been convicted. We continue to call for her release.”

The United Nations secretary general, Ban Ki-moon, called on the ruling junta to “immediately and unconditionally release Daw Aung San Suu Kyi and to engage with her without delay as an essential partner in the process of national dialogue and reconciliation.”

France called a meeting of the United Nations Security Council to adopt a statement condemning the sentence, but the talks were adjourned until Wednesday, Reuters reported.

The European Union said it would seek to impose new economic sanctions on Myanmar, and Britain demanded an arms embargo. Sanctions like these have been largely symbolic over the years as Myanmar has continued to trade with China and its neighbors in Southeast Asia.

Meanwhile, the sentencing of Mr. Yettaw drew condemnation and concern from the United States. Mr. Obama’s statement called it “out of proportion with his actions.”

Philip J. Crowley, a State Department spokesman, called the sentence “cruel,” and said, “We remain gravely concerned about his health and the harsh sentence imposed upon him.”

His lawyer, U Khin Maung Oo, speaking by telephone after the verdict, said, “He’s a man of mental and physical courage.”

The lawyer said he would file an appeal on behalf of Mr. Yettaw, explaining, “He only asked me to try to do anything that ‘will help me to get out of here.’ ”

Looking composed and engaged after the verdict was read, Mrs. Aung San Suu Kyi thanked the two dozen foreign diplomats in the courtroom for their support and, according to one, said, “Hopefully we can work closely together for the peace and prosperity of my country and the rest of the world.” The election set for next year will for the first time put a civilian face on the military leadership that has ruled Myanmar, formerly Burma, since a coup in 1962 while also ensuring the continued dominance of the military in politics and power.

In the eyes of the generals, it would supersede an election in 1990 that they annulled when Mrs. Aung San Suu Kyi’s party, the National League for Democracy, won more than 80 percent of the seats. Many of the party’s winning candidates have since been imprisoned.

When the judge read aloud the sentence of hard labor, according to a European diplomat, there was a stunned reaction.

Then the home minister, Gen. Maung Oo, stepped forward and read a statement of commutation from the junta’s leader, Senior Gen. Than Shwe, whose personal antagonism toward Mrs. Aung San Suu Kyi has become legendary among analysts of Myanmar.

The senior general said he was reducing the sentence because of Mrs. Aung San Suu Kyi’s heritage as the daughter of the country’s assassinated independence hero, U Aung San; in the interests of the country’s “peace and tranquillity”; and in a gesture of better personal relations, the European diplomat said.

“It was carefully calibrated to deliver the word that ‘I am a reasonable man and I’ve listened to what the international community said and here is proof of this,’ ” the diplomat said. SETH MYDANS in nytimes.com, August 11