Earlier police cordoned off the prison behind barricades of barbed wire, following a call from opposition supporters for protesters to gather outside.
But Khin Ohmar, an exiled political activist based in neighbouring Thailand, told Al Jazeera that about 500 people had gathered outside the jail, including some US embassy officials and youth members of Suu Kyi's National League for Democracy (NLD).
According to witnesses, police had arrested several members of the NLD.
"More and more people are coming out to protest, but as more people come out, security is being increased," Ohmar said.
"It is very likely, knowing the nature of this regime, that the demonstrations may get violent."
Former student demonstrators and monks involved in 2007 street protests that were crushed by the military said in a joint statement that they would "oppose this latest atrocity using any means until Daw Aung San Suu Kyi is freed".
"Daw" is an honorific term used in Myanmar.
Critics say the charges against Suu Kyi are aimed at keeping her in detention ahead of the government's promised multi-party elections in 2010.
The country's military rulers have detained the 63-year-old for more than 13 of the past 20 years, much of that time at her lakeside Yangon home guarded by police.
The latest charges apparently stem from an incident earlier this month in which an American man, John Yettaw, was arrested for allegedly swimming across a lake to secretly enter her home.
According to police he stayed there for two days before trying to swim back.
|Myanmar exiles have been holding protests against the trial [Reuters]
As the trial began on Monday morning, the ambassadors of Great Britain, France, Germany and Italy as well as an Australian diplomat were barred from entering the prison, but the US consul was allowed into the prison compound since Yettaw, a US citizen, was on trial along with Suu Kyi.
Drake Wiesert, the US embassy spokesman, said it was unclear if the consul would be allowed to attend the court proceedings or whether Yettaw was represented by a lawyer.
Yettaw had been charged with immigration violations, encouraging others to break the law and entering a restricted area.
The motives for his attempt to meet Suu Kyi are unclear.
Yettaw swam to her home in November last year but the Nobel Peace laureate refused to see him.
He tried again on the night of May 3 and according to a police complaint: "This time, Daw Aung San Suu Kyi allowed him to stay at her residence until the night of May 5, 2009, spoke with him and provided him food and drinks."
Kyi Win, a lawyer representing Suu Kyi, said his client told Yettaw to leave, but he refused.
She did not report him to authorities because "she did not want anybody to get into trouble because of her", Kyi Win added.
Jared Genser, an international lawyer also representing the opposition leader, said in a statement that it was "highly doubtful justice will be served" in the trial.
He also accused the military government of failing to provide sufficient security around Aung San Suu Kyi's home, leading to Yettaw breaking into the property.
Western governments, the United Nations and human rights groups have condemned the charges against Suu Kyi and called for her immediate release.
The European Union has said that sanctions against the Myanmar government should be increased due to the trial.
"It's not the moment to lower the sanctions, it's the moment to increase them," Javier Solana, the EU foreign policy chief, told reporters in Brussels on Monday.
"We are ready to discuss the reinforcement of the sanctions," added Jan Kohout, the Czech Foreign Minister, whose country holds the rotating EU presidency.
"[But] First we will call on the Burmese [Myanmar] authorities to release Aung San Suu Kyi."
|Police cordoned off the prison following a call from the opposition for protests [AFP]
In April, EU foreign ministers extended sanctions against Myanmar for another year, but said they were ready to ease them and hold talks if there was democratic progress.
Neighbouring Southeast Asian countries have also criticised Suu Kyi's trial.
Hassan Wirajuda, the Indonesian foreign minister, said the trial sent "the wrong message".
"In the past 10 years, Daw San Suu Kyi was detained arbitrarily. Actually, we were hoping that if there is any trial, it should decide to release Aung San Suu Kyi, and not to extend here detention," he said.
In unusually sharp criticism from a Southeast Asian nation, Alberto Romulo, the Philippine foreign affairs secretary, said on Sunday that his government was "deeply troubled and outraged'' over the "trumped-up charges'' against Suu Kyi.
"We urge the government of Myanmar to resolve the matter speedily and to release Aung San Suu Kyi immediately and unconditionally," he said.
Members of the 10-country Association of Southeast Asian Nations (Asean), which includes Myanmar, normally refrain from criticising one another.
However, despite the international outcry, state-controlled media in Myanmar has said nothing about the trial.