Bangkok—Myanmar will hold its first election in two decades on Nov. 7, the ruling junta announced on Friday, setting a date for a vote that is seen as a means of legitimizing military power within the format of civilian rule.The main opposition party, the National League for Democracy, is boycotting the election, saying the electoral rules are unfair and restrictive. Its leader, Daw Aung San Suu Kyi, a Nobel Peace laureate who has spent most of the last 20 years under house arrest, is legally barred from running in the election because she is under government detention.

The party was officially disbanded in May because it refused to register for the campaign.

The brief election announcement, carried on government radio and television stations, said, “Multiparty general elections for the country’s parliament will be held Sunday, Nov. 7.” It gave political parties until the end of this month to submit their candidate lists.

The timing of the election gives parties only a short time to recruit candidates and mount campaigns in what one Burmese exile commentator, Win Tin, called “a calculated political ambush.”

The United States and other Western nations have condemned the election as undemocratic. After a two-day visit in early May, Assistant Secretary of State Kurt M. Campbell said he was “profoundly disappointed” over the preparations for the election.

“What we have seen to date leads us to believe that these elections will lack international legitimacy,” he said.

Although the future Parliament ostensibly marks a shift to civilian government, it is heavily weighted toward the military, which has held power for the past half century. One quarter of the 440 seats will be reserved for active members of the military, which will allow them to control important ministries including those responsible for justice, defense and internal affairs.

Many other candidates are former military officers. In April, the prime minister and 22 other ministers retired from their military posts to run for office as civilians.

In addition, a new constitution creates a powerful National Defense and Security Council, controlled by the military commander in chief, that has the power to overrule the civilian government.

Electoral rules also favor the junta, placing tight restrictions on campaigns and public statements that criticize the government.

At least 40 parties have registered to run, including the Union Solidarity and Development Party, regarded as a vehicle for the junta’s candidates, which is believed to have received state money and special privileges.

The now-dissolved National League for Democracy won the last election in 1990 by an overwhelming margin but was denied its seats by the military.

The new election is scheduled to take place shortly before Mrs. Aung San Suu Kyi’s latest term of house arrest expires. The fact that her late husband was a foreigner also prohibits her from running.

Other prisoners, including an estimated 2,000 political prisoners, are also barred from taking part.

A breakaway faction of Mrs. Aung San Suu Kyi’s party, the National Democratic Force, is taking part in the election despite her objections. It has squabbled with some other members of the party.

Most other parties represent ethnic minority groups with specific local agendas. But pro-democracy parties are facing the most difficulties.

A prominent opposition candidate and former political prisoner, Phyo Min Thein, recently resigned as head of his Union Democratic Party because of what he called an “unfavorable pre-election environment.”

“My resignation is proof to the international community that the forthcoming elections will not be free and fair,” he said in a statement.