"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.
"For the time being rule of law is very weak. There are still arbitrary arrests and there is no fair trial," Bo Kyi added.
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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.
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.