Nant Bwa Bwa Phan, the UK representative of the Karen National Union - a group of seven million Burmese people - said the release of the pro-democracy leader was "a public relations exercise to fool the international community".
"We are very happy and overjoyed by her release, but extremely worried that people will think there has been a significant change in Burma, that this is democratisation," she said. "That is not the government's intention.
"We don't know when she will be detained again - it could happen any time, like it did in 2003. She is now in a very serious situation."
Speaking at an Amnesty International event in central London, Ms Phan, who fled Burma 15 years ago to escape attacks on her ethnic community, added that the true extent of the Burmese conflict remained unknown. "Things that happen on the ground are very different to what the rest of the world sees," she said.
"Life for many people is far from the happiness yesterday in Rangoon - people are struggling to get on with their lives.
"To be able to have a genuine freedom and democracy in Burma, the way forward is dialogue - we need to put pressure on the government to make that dialogue start soon. Aung San Suu Kyi herself said that this isn't the end of the struggle - this is only the beginning."
Verity Coyle, Amnesty's campaign manager in Burma, said: "People gathering and smiling in the streets is something we haven't seen for a long time in Burma - there was just too much fear.
"Concern now is for the security of Aung San Suu Kyi and that of every person who went to be part of the moment of her release.
"The world shouldn't be fooled - there are still more than 2,200 political prisoners in the most despicable conditions all over Burma."
Representatives of Amnesty UK, which is running a campaign to aid political communication in Burma through radio, said the international community now has a role to play in ensuring Aung San Suu Kyi's safety.