Activists on Tuesday urged Myanmar 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 during a visit to London.
Pro-democracy campaigners, however, have accused the former junta premier of using a series of headline-grabbing amnesties to win 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), who estimates there are still more than 150 political prisoners behind bars.
He said it would be difficult for the government to keep its promise to release all political detainees as dozens more activists are facing trial, including people arrested 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 ex-military man has been lauded by foreign governments for reforms including welcoming democracy champion Aung San 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 United States has also lifted most embargoes and foreign companies are now eager to enter the resource-rich nation, with its perceived frontier market of some 60 million potential consumers.
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.
"Difficult talks will follow and hard compromises will need to be made. But it must be done."
Skirmishes between government troops and rebels continue, particularly in the northern state of Kachin where the two sides have agreed to try to calm military tensions but have yet to sign a proper ceasefire.
Activists also say human rights violations including forced labour, arbitrary arrest, detention and land confiscation continue in areas such as eastern Karen State.
"We know from our experience that we cannot trust the Burmese Army," said Htoo Ku Hsa Say of the Karen Community Association UK, calling on Thein Sein to withdraw all his troops from the war-torn state bordering Thailand.
British Prime Minister David Cameron urged Thein Sein to defend human rights during talks in London.
Welcoming the Myanmar leader on the red carpet outside his 10 Downing Street office, Cameron said he was "very pleased" to see Thein Sein on his "historic visit".
But Cameron added: "As well as the continuation of your reform process, we are also very keen to see greater action in terms of promoting human rights and dealing with regional conflicts.
"We are particularly concerned about what has happened in Rakhine province and the Rohingya Muslims."
Thein Sein promised to take a "zero tolerance approach" to people who "fuel ethnic hatreds" following a recent wave of religious bloodshed.
Buddhist-Muslim clashes in the western state of Rakhine last year left about 200 people dead, mostly Rohingya minority Muslims who are denied citizenship by Myanmar.
Sectarian strife has since spread to other parts of the country, including central Myanmar where at least 44 people were killed in March.