Activists yesterday urged Myanmar's president, Thein Sein, to "turn his words into action" after the former general promised to free all political prisoners by the end of the year.
"I guarantee to you that by the end of this year there will be no prisoners of conscience in Myanmar," Thein Sein said on Monday during a visit to London.
Pro-democracy campaigners have accused the president of using a series of headline-grabbing amnesties to secure foreign aid and investment.
"President Thein Sein is very good at PR but he needs to turn his words into action," said Bo Kyi of the Thailand-based Assistance Association for Political Prisoners (Burma), which estimates there are still more than 150 political prisoners behind bars.
He said it would be difficult for the government to keep its promise while dozens of activists are facing trial, some for protesting without permission.
"For the time being rule of law is very weak. There are still arbitrary arrests and there is no fair trial," Bo Kyi added.
The military junta, which ruled for decades until 2011, had long denied the existence of political prisoners. But hundreds of dissidents have been freed since Thein Sein took power in March 2011.
Last November he announced a review of all "politically concerned" cases.
The opposition party of Nobel Peace laureate Aung San Suu Kyi, the National League for Democracy, estimates that about 100 political prisoners continue to languish in Myanmar's prisons.
"We welcome what the president said. We also want them all to be freed," party spokesman Nyan Win said.
Thein Sein has been praised for reforms including welcoming Suu Kyi and her political party into parliament.
In response the European Union has scrapped most sanctions, except for an arms embargo, and readmitted Myanmar to its trade preference scheme.
The US has also lifted most embargoes and foreign companies are now eager to enter the resource-rich nation.
Thein Sein has also reached tentative peace deals with the major armed ethnic minority rebel groups, and he voiced optimism in London about ending decades of conflict.
"Very possibly over the coming weeks we will have a nationwide ceasefire and the guns will go silent everywhere in Myanmar for the very first time in over 60 years," he said.