The release of Burmese pro-democracy leader Aung San Suu Kyi was "long overdue" and her detention a "travesty", Prime Minister David Cameron has said.
He also called her "an inspiration for all of us who believe in freedom of speech, democracy and human rights".
Former Prime Minister Gordon Brown said she was "the world's most renowned and courageous prisoner of conscience".
Burma Campaign UK urged the immediate release of 2,202 political prisoners.
Nobel Peace Prize winner Ms Suu Kyi has been detained for 15 of the past 21 years.
Mr Cameron said: "This is long overdue. Aung San Suu Kyi is an inspiration for all of us who believe in freedom of speech, democracy and human rights.
"Her detention was a travesty, designed only to silence the voice of the Burmese people. Freedom is Aung San Suu Kyi's right. The Burmese regime must now uphold it."
Mr Brown said Ms Suu Kyi's release would bring "joy round the world".
But he said her freedom would only be a "partial victory, because her liberation and that of the Burmese people will not be complete until she is able to take up her position as the rightful leader of her country".
Ms Suu Kyi, 65, lived in London and Oxford while she raised her two sons with her late husband, British scholar Michael Aris.
He died of prostate cancer in 1999 aged 53.
Her younger son Kim, 33, who lives in the UK and has not seen his mother in 10 years, is currently in Bangkok, Thailand. He spoke to his mother by telephone after her release. Her eldest son, Alexander, is understood to live in the US.
Foreign Secretary William Hague said Ms Suu Kyi's detention had been "deeply unjust".
He added: "Her fortitude in the face of this outrage has been inspirational. I welcome news of her release. She must now be allowed to assume a role of her choosing in the political life of her country without further hindrance or restriction.
"Last week's sham elections will not bring peace and prosperity to Burma. The regime now needs to release the other 2,100 political prisoners and begin a genuine dialogue with Aung San Suu Kyi and all opposition and ethnic groups.
"These remain the crucial first steps to solving Burma's many problems and addressing the pressing needs of its people."
Labour Party leader Ed Miliband said Ms Suu Kyi's release was "an important milestone for the people of Burma as they seek to end the repression under which they have lived for so long.
"However, we must see this as only one step in a long journey for Burma. Governments must continue to keep pressure on the military regime to release the more than 2200 political prisoners that they still hold."
Burma Campaign UK warned that her release should not be interpreted as a sign that democratic reform was on the way in Burma.
Ms Suu Kyi's brother-in-law Adrian Phillips said her release was a "happy day".
"We are obviously very pleased if it means we can contact her again after so many years of silence," he said.
"The last time I spoke to her was when her husband died in 1999. There are all sorts of family matters that we haven't been able to talk to her about. She has a grand-daughter, Jasmine, who she has never seen."
Zoya Phan, international co-ordinator at Burma Campaign UK, said: "The release of Aung San Suu Kyi is about public relations, not democratic reform.
"I am thrilled to see our democracy leader free at last, but the release is not part of any political process, instead it is designed to get positive publicity for the dictatorship after the blatant rigging of elections on 7 November.
"We must not forget the thousands of other political prisoners still suffering in Burma's jails."
But she also told the BBC: "We don't know how long she will be free, because in the past she was freed, and then she was re-arrested again. In 1995 she was freed with conditions."
Amnesty International also welcomed the release of Aung San Suu Kyi, but also called on the government of Burma to immediately release "all of the prisoners of conscience in the country".
Secretary General Salil Shetty said: "While Aung San Suu Kyi's release is certainly welcome, it only marks the end of an unfair sentence that was illegally extended, and is by no means a concession on the part of the authorities.
"The fact remains that authorities should never have arrested her or the many other prisoners of conscience in Burma in the first place, locking them out of the political process."