"Free, Free Daw Aung San Suu Kyi," they shouted in English, waving banners and placards on the day the 63-year-old Nobel Peace Prize winner went on trial in Myanmar.

The activists denounced the military dictatorship in Myanmar for what they called the "unjust detention".

The trial comes just days after she was imprisoned at a "guest house" inside Yangon's Insein prison compound on charges of breaching security laws.

The charges relate to the illegal visit of US national John Yettaw who earlier this month used a pair of homemade flippers to swim across a lake to the residence where Aung San Suu Kyi has been kept in virtual isolation for most of the past 19 years.

Two of her aides are being tried with her.

The freedom icon, who had been due to end her latest sentence later this month, faces another five years without freedom.

Yettaw is also in court over the incident.

Protesters at the two-hour afternoon rally held up protest posters, banners and portraits of Aung San Suu Kyi outside the embassy, located in a narrow one-lane street in Tokyo, as about a dozen police stood by.

The protesters threw letters over the embassy's iron gate along with a placard with two crossed-out pictures of Myanmar's leader General Than Shwe.

"We, democratic Burmese (Myanmar) forces in Japan, wish that the Japanese government, the United Nations and the international community immediately take action against the junta's outrageous act," said activist Than Swe.

"If the problem escalates, there is a possibility that it might lead to civil war," warned one letter from the Japan branch of the National Council of the Union of Burma, urging instead "a peaceful solution."

Japanese Buddhist priest Akira Egami told the rally: "We, the Buddhists, want the government of Myanmar, a Buddhist nation, to listen to our prayers for the avoidance of violence and immediately release Madame Aung San Suu Kyi."

Japan, Myanmar's main donor among the world's major industrial democracies, in 2003 suspended most assistance other than emergency aid and some training funds.

Tokyo further cut its aid after the regime cracked down on pro-democracy demonstrations led by Buddhist monks in 2007, but it refuses to join its Western allies in imposing sanctions on Myanmar, formerly known as Burma.

Than Swe, 46, head of the Japanese branch of the Burma Democratic Action Group, said the protesters included holdovers from the past pro-democracy movement in Myanmar and ordinary people who came to Japan to work or study.

"I came to Japan in 1989 after escaping arrest in the 1988 pro-democracy demonstrations when my buddies were put into jail," he said.

He said his group has branches in Japan and Thailand with covert members based in Myanmar and representatives in the United States.